Switch build-backend to flit_core + use License-Expression for project metadata (#59)

MANIFEST.in

diff --git a/MANIFEST.in b/MANIFEST.in
deleted file mode 100644
index d0bc2e5..0000000
--- a/MANIFEST.in
+++ /dev/null
@@ -1,6 +0,0 @@
-# files necessary for testing sdist
-graft tests
-include tox.ini
-
-global-exclude *.py[cod]
-global-exclude .DS_Store

pyproject.toml

diff --git a/pyproject.toml b/pyproject.toml
index de8ddc4..2395ca3 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [build-system]
-requires      = ["setuptools>=75.0"]
-build-backend = "setuptools.build_meta"
+requires      = ["flit_core>=3.11,<4"]
+build-backend = "flit_core.buildapi"
 
 [project]
 name        = "mypy_extensions"
@@ -10,12 +10,11 @@
 authors     = [
     {name = "The mypy developers", email = "jukka.lehtosalo@iki.fi"}
 ]
-license     = {text = "MIT"}
+license     = "MIT"
 classifiers = [
     "Development Status :: 5 - Production/Stable",
     "Environment :: Console",
     "Intended Audience :: Developers",
-    "License :: OSI Approved :: MIT License",
     "Programming Language :: Python :: 3",
     "Programming Language :: Python :: 3.8",
     "Programming Language :: Python :: 3.9",
@@ -30,5 +29,5 @@
 [project.urls]
 Homepage = "https://github.com/python/mypy_extensions"
 
-[tool.setuptools]
-py-modules = ["mypy_extensions"]
+[tool.flit.sdist]
+include = ["tox.ini", "*/*test*.py"]