gh-148663: Document that IllegalMonthError inherits from both ValueError and IndexError

[EoinTrial]

The documentation for calendar.IllegalMonthError describes it only as "a subclass of ValueError", but the source code defines it as:

class IllegalMonthError(ValueError, IndexError):

The IndexError base class is intentional for backwards compatibility (see the source comment in Lib/calendar.py), but this is not mentioned in the docs. Users catching IndexError for bad month numbers won't know their code still works after Python 3.13.

• Issue: calendar.IllegalMonthError docs omit IndexError base class #148663

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📚 Documentation preview 📚: https://cpython-previews--148664.org.readthedocs.build/

[python-cla-bot]

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• 45000144+EoinTrial@users.noreply.github.com

[picnixz]

1. Can you add a versionchanged directive indicating that this is now inheriting ValuerError and IndexError since 3.13 (I did not check when it was changed so pick the correct version).
2. Reword the NEWS entry
3. Add a test to ensure that we do not remove that double inheritance.

[EoinTrial]

@picnixz Thanks for taking a look! I've added the versionchanged:: 3.12 directive, rewritten the NEWS entry, and added test_illegal_month_error_bases to verify the dual inheritance.

[picnixz]

Suggested change

	:exc:`IllegalMonthError` is now also a subclass of :exc:`IndexError`.
	:exc:`IllegalMonthError`is now also a subclass of :exc:`ValueError`. Newer code should avoid catching :exc:`IndexError`.

And make that paragraph fit on 80 chars.

[picnixz]

There is a helper assertIsSubclass