Django v5 support

[aarondewindt]

I accidentally upgraded my backend image to django v5 and came across an issue. I'm reverting back to v4, but figured I'd start an issue to maybe update this package so it supports django v5 in the future.

The breaking change is:

The django.utils.timezone.utc alias to datetime.timezone.utc is removed.

Used at: https://github.com/myyang/django-unixtimestampfield/blob/f33a8aea42f477a8462331e6b62f121428a93447/unixtimestampfield/fields.py#L97C55-L97C55

There might be more, but this is the only one I've found so far.