gh-151770: Fix datetime.fromisoformat() assertion on an out-of-range month with a 24:00 time

[tonghuaroot]

datetime.fromisoformat() reached days_in_month() with month == 0 when the input combined an out-of-range (below 1) month with a 24:00 time, for example "2009-00-01T24:00:00". On a --with-pydebug build this tripped assert(month >= 1) in days_in_month (Modules/_datetimemodule.c); the pure-Python implementation in Lib/_pydatetime.py raised AssertionError on the same input. On a release build both already raised ValueError.

The 24:00 midnight-rollover guard validated only the upper month bound (month <= 12) before the days_in_month() call, and the parser does not range-check the month before that point, so a month below 1 slipped through. The matching upper-bound input "2009-13-01T24:00:00" was already rejected by the month <= 12 check.

This adds the missing month >= 1 lower bound to the guard in both implementations (mirroring the existing check_date_args validation), so an out-of-range month consistently raises ValueError. A regression test is added next to the existing out-of-range-month case in datetimetester.py.

• Issue: datetime.fromisoformat() hits a debug-build assertion on an out-of-range month combined with a 24:00 time #151770

[StanFromIreland]

Just a few little nits, the core change is correct.

While we're dealing with this assert, can you please update it (on L58) like so:

- assert 1 <= month <= 12, month
+ assert 1 <= month <= 12, f"month must be in 1..12, not {month}"

[StanFromIreland]

Suggested change

	Fix :meth:`datetime.datetime.fromisoformat` raising an unexpected error on an
	out-of-range month combined with a ``24:00`` time, such as
	``"2009-00-01T24:00:00"``. It now consistently raises :exc:`ValueError`.
	Fix :meth:`datetime.datetime.fromisoformat` raising :exc:`AssertionError`
	instead of :exc:`ValueError` for an out-of-range month combined with a
	``24:00`` time.

[StanFromIreland]

Suggested change

	"2009-00-01T24:00:00", # Month out of range (below)
	"2009-00-01T24:00:00", # Month below range

[tonghuaroot]

Thanks @StanFromIreland — applied all three.

[StanFromIreland]

LGTM