gh-90385: Add `pathlib.Path.walk()` method (GH-92517)

Doc/library/pathlib.rst

Lib/pathlib.py

Lib/test/support/os_helper.py

Lib/test/test_pathlib.py

...ext/Library/2022-05-10-16-30-40.gh-issue-90385.1_wBRQ.rst