Add NoReturn to mypy_extensions (#2637)

mypy_extensions.py

diff --git a/mypy_extensions.py b/mypy_extensions.py
index 8305902..26e568c 100644
--- a/mypy_extensions.py
+++ b/mypy_extensions.py
@@ -91,3 +91,7 @@
     The latter syntax is only supported in Python 3.6+, while two other
     syntax forms work for Python 2.7 and 3.2+
     """
+
+
+# Return type that indicates a function does not return
+class NoReturn: pass