Use of timezone in Time.datetime #15311

What is the problem this feature will solve?

Python in trying to move away from datetime that have not explicitly set a timezone, raising warning for, e.g., datetime.utcnow() which caused problems for our tests for python 3.12 (see #14784 and the discussion in #15298). This may mean that output from time.to_datetime() by default produces output that cannot easily be calculated with.

Describe the desired outcome

To at least communicate that our scale="utc" times have timezone="utc", i.e., make TimeDatetime as informative as possible. For scale="utc", it could set timezone="utc" (if no explicit timezone is given by calling to_datetime(timezone), for other scales it still only makes sense to use timezone=None.

EDIT: in principle, we could also store any timezone the user has given on Time, but this would seem to add a lot of complexity for little benefit for anything astronomers actually need Time for.

Additional context

The actual change would be rather minimal. From #15298 (comment), it would require changes at

Essentially, one would change those lines to,

        if self._scale == "utc":
            if timezone is None:
                timezone = timezone.utc
        else:
            if timezone is not None:
                raise ScaleValueError(
                    f"scale is {self._scale}, must be 'utc' when timezone is supplied."
                )