http_date_to_dt() should return a timezone-aware datetime

[vytas7]

In line with the upstream deprecation of utcnow() in Python 3.12, I think that we should change http_date_to_dt() to return a timezone-aware datetime. Since HTTP dates are normally in GMT, we could use our own GMT zone, or simply use UTC.

This is a breaking change of relatively low value, so unfortunately we won't be able to prioritize it for 4.0. (And we need a longer deprecation window in any case.)