bpo-35486: Note Py3.6 import system API requirement change (GH-11540) (GH-11588)

bpo-35486: Note Py3.6 import system API requirement change (GH-11540) (GH-11588)

While the introduction of ModuleNotFoundError was fully backwards
compatible on the import API consumer side, folks providing alternative
implementations of `__import__` need to make an update to be
forward compatible with clients that start relying on the new subclass.

https://bugs.python.org/issue35486
(cherry picked from commit cee29b46a19116261b083dc803217aa754c7df40)

Co-authored-by: Nick Coghlan <ncoghlan@gmail.com>