docs: correct outdated MappingProxyType docstrings (GH-30281)

The docstrings for MappingProxyType's keys(), values(), and items()
methods were never updated to reflect the changes that Python 3 brought
to these APIs, namely returning views rather than lists.
(cherry picked from commit 2d10fa9bc4cf83c5e5dd73decc9a138d6d247374)

Co-authored-by: Joshua Bronson <jabronson@gmail.com>