DateTime.combine should default to time.tzinfo

[RazerM]

Fixes #19

[RazerM]

Some context for the sentinel I used here:

The Python implemention of CPython's datetime module uses True for the sentinel, but that is not part of the API and doesn't work with the builtin module:

❯ uv run --isolated --no-project -p 3.14 python
Python 3.14.0 (main, Oct 28 2025, 12:03:45) [Clang 20.1.4 ] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import _datetime
>>> import _pydatetime
>>> _pydatetime.datetime.combine(_pydatetime.date.today(), _pydatetime.time(tzinfo=_pydatetime.UTC), True)
datetime.datetime(2025, 11, 11, 0, 0, tzinfo=datetime.timezone.utc)
>>> _datetime.datetime.combine(_datetime.date.today(), _datetime.time(tzinfo=_datetime.UTC), True)
Traceback (most recent call last):
  File "<python-input-3>", line 1, in <module>
    _datetime.datetime.combine(_datetime.date.today(), _datetime.time(tzinfo=_datetime.UTC), True)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: tzinfo argument must be None or of a tzinfo subclass, not type 'bool'

[glyph]

I think there might be a slightly better way to express this, but the update is clearly correct