gh-140715: Add %D format code support to strptime()

[jyalim]

Incrementally builds C99+ datetime support with addition of %D with single line of code. Also adds tests and updates documentation.

Continued effort from PR: #140647.

Documentation drops (0) footnote, changes example date to increase readability for non-US readers.

Passed all tests (especially changes to new one).

Passed patchcheck.

• Issue: Update str*time to C11 format codes #140715

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📚 Documentation preview 📚: https://cpython-previews--144819.org.readthedocs.build/

[StanFromIreland]

You have to update test_strptime_exception_context to use a different (invalid) format code.

Also please add it here too:

cpython/Lib/test/test_time.py

Line 224 in 14cbd0e

for directive in ('a', 'A', 'b', 'B', 'c', 'd', 'H', 'I',

[jyalim]

Thank you, those critical changes were made!