Improve/add license headers (#176)

diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 8173b11..9c9cf2f 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -3,8 +3,8 @@
   rev: 79b8e4f273301e412602c856ddbc0c066765c98c  # frozen: v1.1.11
   hooks:
   - id: insert-license
-    files: '^src/.+\.py$'
-    args: ['--license-filepath', 'LICENSE']
+    files: '^src/.+\.py$|^tests/.+\.py$'
+    args: ['--license-filepath', 'LICENSE-HEADER']
 - repo: https://github.com/executablebooks/mdformat
   rev: a5f167d00d7a1990e69ee02efacc15d6136ea1aa  # frozen: 0.7.13
   hooks:
diff --git a/LICENSE-HEADER b/LICENSE-HEADER
new file mode 100644
index 0000000..aba78dd
--- /dev/null
+++ b/LICENSE-HEADER
@@ -0,0 +1,3 @@
+SPDX-License-Identifier: MIT
+SPDX-FileCopyrightText: 2021 Taneli Hukkinen
+Licensed to PSF under a Contributor Agreement.
diff --git a/src/tomli/__init__.py b/src/tomli/__init__.py
index d0d8ecf..34b0b40 100644
--- a/src/tomli/__init__.py
+++ b/src/tomli/__init__.py
@@ -1,24 +1,6 @@
-# MIT License
-#
-# Copyright (c) 2021 Taneli Hukkinen
-#
-# Permission is hereby granted, free of charge, to any person obtaining a copy
-# of this software and associated documentation files (the "Software"), to deal
-# in the Software without restriction, including without limitation the rights
-# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
-# copies of the Software, and to permit persons to whom the Software is
-# furnished to do so, subject to the following conditions:
-#
-# The above copyright notice and this permission notice shall be included in all
-# copies or substantial portions of the Software.
-#
-# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
-# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
-# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
-# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
-# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
-# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
-# SOFTWARE.
+# SPDX-License-Identifier: MIT
+# SPDX-FileCopyrightText: 2021 Taneli Hukkinen
+# Licensed to PSF under a Contributor Agreement.
 
 __all__ = ("loads", "load", "TOMLDecodeError")
 __version__ = "2.0.0"  # DO NOT EDIT THIS LINE MANUALLY. LET bump2version UTILITY DO IT
diff --git a/src/tomli/_parser.py b/src/tomli/_parser.py
index c000196..65da389 100644
--- a/src/tomli/_parser.py
+++ b/src/tomli/_parser.py
@@ -1,24 +1,6 @@
-# MIT License
-#
-# Copyright (c) 2021 Taneli Hukkinen
-#
-# Permission is hereby granted, free of charge, to any person obtaining a copy
-# of this software and associated documentation files (the "Software"), to deal
-# in the Software without restriction, including without limitation the rights
-# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
-# copies of the Software, and to permit persons to whom the Software is
-# furnished to do so, subject to the following conditions:
-#
-# The above copyright notice and this permission notice shall be included in all
-# copies or substantial portions of the Software.
-#
-# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
-# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
-# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
-# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
-# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
-# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
-# SOFTWARE.
+# SPDX-License-Identifier: MIT
+# SPDX-FileCopyrightText: 2021 Taneli Hukkinen
+# Licensed to PSF under a Contributor Agreement.
 
 from __future__ import annotations
 
diff --git a/src/tomli/_re.py b/src/tomli/_re.py
index 8603fae..994bb74 100644
--- a/src/tomli/_re.py
+++ b/src/tomli/_re.py
@@ -1,24 +1,6 @@
-# MIT License
-#
-# Copyright (c) 2021 Taneli Hukkinen
-#
-# Permission is hereby granted, free of charge, to any person obtaining a copy
-# of this software and associated documentation files (the "Software"), to deal
-# in the Software without restriction, including without limitation the rights
-# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
-# copies of the Software, and to permit persons to whom the Software is
-# furnished to do so, subject to the following conditions:
-#
-# The above copyright notice and this permission notice shall be included in all
-# copies or substantial portions of the Software.
-#
-# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
-# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
-# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
-# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
-# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
-# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
-# SOFTWARE.
+# SPDX-License-Identifier: MIT
+# SPDX-FileCopyrightText: 2021 Taneli Hukkinen
+# Licensed to PSF under a Contributor Agreement.
 
 from __future__ import annotations
 
diff --git a/src/tomli/_types.py b/src/tomli/_types.py
index ad1d810..d949412 100644
--- a/src/tomli/_types.py
+++ b/src/tomli/_types.py
@@ -1,24 +1,6 @@
-# MIT License
-#
-# Copyright (c) 2021 Taneli Hukkinen
-#
-# Permission is hereby granted, free of charge, to any person obtaining a copy
-# of this software and associated documentation files (the "Software"), to deal
-# in the Software without restriction, including without limitation the rights
-# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
-# copies of the Software, and to permit persons to whom the Software is
-# furnished to do so, subject to the following conditions:
-#
-# The above copyright notice and this permission notice shall be included in all
-# copies or substantial portions of the Software.
-#
-# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
-# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
-# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
-# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
-# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
-# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
-# SOFTWARE.
+# SPDX-License-Identifier: MIT
+# SPDX-FileCopyrightText: 2021 Taneli Hukkinen
+# Licensed to PSF under a Contributor Agreement.
 
 from typing import Any, Callable, Tuple
 
diff --git a/tests/__init__.py b/tests/__init__.py
index 097446a..4e37361 100644
--- a/tests/__init__.py
+++ b/tests/__init__.py
@@ -1,3 +1,7 @@
+# SPDX-License-Identifier: MIT
+# SPDX-FileCopyrightText: 2021 Taneli Hukkinen
+# Licensed to PSF under a Contributor Agreement.
+
 __all__ = ("tomllib",)
 
 # By changing this one line, we can run the tests against
diff --git a/tests/burntsushi.py b/tests/burntsushi.py
index 39b23a3..71228c6 100644
--- a/tests/burntsushi.py
+++ b/tests/burntsushi.py
@@ -1,3 +1,7 @@
+# SPDX-License-Identifier: MIT
+# SPDX-FileCopyrightText: 2021 Taneli Hukkinen
+# Licensed to PSF under a Contributor Agreement.
+
 """Utilities for tests that are in the "burntsushi" format."""
 
 import datetime
diff --git a/tests/test_data.py b/tests/test_data.py
index f6aafbf..d73ff7c 100644
--- a/tests/test_data.py
+++ b/tests/test_data.py
@@ -1,3 +1,7 @@
+# SPDX-License-Identifier: MIT
+# SPDX-FileCopyrightText: 2021 Taneli Hukkinen
+# Licensed to PSF under a Contributor Agreement.
+
 import json
 from pathlib import Path
 import unittest
diff --git a/tests/test_error.py b/tests/test_error.py
index 5c998da..e2ada25 100644
--- a/tests/test_error.py
+++ b/tests/test_error.py
@@ -1,3 +1,7 @@
+# SPDX-License-Identifier: MIT
+# SPDX-FileCopyrightText: 2021 Taneli Hukkinen
+# Licensed to PSF under a Contributor Agreement.
+
 import unittest
 
 from . import tomllib
diff --git a/tests/test_misc.py b/tests/test_misc.py
index 9401dae..76fa590 100644
--- a/tests/test_misc.py
+++ b/tests/test_misc.py
@@ -1,3 +1,7 @@
+# SPDX-License-Identifier: MIT
+# SPDX-FileCopyrightText: 2021 Taneli Hukkinen
+# Licensed to PSF under a Contributor Agreement.
+
 import copy
 import datetime
 from decimal import Decimal as D