bpo-34903: Document that some strptime formats only require 1 digit (GH-14149)

bpo-34903: Document that some strptime formats only require 1 digit (GH-14149)

For datetime.datetime.strptime(), the leading zero for some two-digit formats is optional.

This adds a footnote to the strftime/strptime documentation to reflect this fact, and adds some tests to ensure that it is true.

bpo-34903
(cherry picked from commit 6b9c204ee77a0de87d6f51a3d4547a18604cef9e)

Co-authored-by: Mike Gleen <mike.gleen@gmail.com>