backported type hints for python >= 3.5
This is a backport of the standard library typing module to Python versions
older than 3.5.

Typing defines a standard notation for Python function and variable type
annotations. The notation can be used for documenting code in a concise,
standard format, and it has been designed to also be used by static and runtime
type checkers, static analyzers, IDEs and other tools.

Maintainer: Renaud Allard <renaud@allard.it>

WWW: https://github.com/python/typing/blob/master/typing_extensions/README.rst
