Add missing checkdepends python-typing_extensions
Currently building sphinx fails:
=================================== FAILURES ===================================
_____________________________ test_restify_Unpack ______________________________
def test_restify_Unpack():
> from typing_extensions import Unpack as UnpackCompat
E ModuleNotFoundError: No module named 'typing_extensions'
tests/test_util/test_util_typing.py:338: ModuleNotFoundError