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

[jyalim]

Incrementally builds C99+ datetime support with addition of %C. Also adds tests (for %C and for %y with %C) and updates documentation.

Continued effort from PR: #140647, #144819, #144896.

Documentation drops (0) footnote and notes that zero-padded century numbers are optional (for centuries 0 to 99).

Passed all tests.

Passed patchcheck.

Issue: #140715

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

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📚 Documentation preview 📚: https://cpython-previews--145955.org.readthedocs.build/

[jyalim]

I also brought this dictionary update into the update above it for greater stylistic consistency.

[jyalim]

Tests and patchcheck re-verified before pushing merged main.

Congrats on the promotion, @StanFromIreland!

[StanFromIreland]

Lint is failing in the CI. And, to be honest I don’t really see any benefit in running patchcheck, I would recommend you run pre-commit (lint) instead. I hope to review in a few days.

[jyalim]

Thank you, @StanFromIreland, all requests were implemented!

[jyalim]

Keeping current with upstream.

[jyalim]

@StanFromIreland I think this is in a good place after the simplification from two months ago.

[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #33974553 | 📁 Comparing db98815 against main (d206d42)

🔍 Preview build

291 files changed · + 9 added · ± 281 modified · - 1 deleted

+Added

• c-api/sentinel.html
• c-api/slots.html
• deprecations/pending-removal-in-3.21.html
• howto/abi3t-migration.html
• library/asyncio-threading.html
• library/asyncio-tools.html
• library/tkinter.fontchooser.html
• library/tkinter.systray.html
• whatsnew/3.16.html

±Modified

• bugs.html
• glossary.html
• index.html
• license.html
• py-modindex.html
• c-api/arg.html
• c-api/bytes.html
• c-api/complex.html
• c-api/concrete.html
• c-api/coro.html
• and 271 more...

-Deleted

• library/asyncio-policy.html

[jyalim]

Thank you, @StanFromIreland, that further strengthened the implementation. All changes have been made, as requested.

[jyalim]

Hi @StanFromIreland, this is ready for another round of review after the implemented changes from May 22.