Add NoReturn to mypy_extensions (#2637)

Author: ddfisher

extensions/mypy_extensions.py

@@ -91,3 +91,7 @@ class Point2D(TypedDict):
     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