Allow using datetime.timezone.utc for UTC timestampts

[adriangb]

It seems that psycopg uses zoneinfo.ZoneInfo to load timezones from Postgres' reported timezone. This makes sense but unfortunately zoneinfo.ZoneInfo('UTC') != datetime.timezone.utc for reasons I don't understand. Since using UTC everywhere is such a common use case, would be reasonable to add a connection level flag to special case loading UTC as datetime.timezone.utc?

[adriangb]

Never mind datetime.datetime(2021, 1, 1, tzinfo=zoneinfo.ZoneInfo('UTC')) == datetime.datetime(2021, 1, 1, tzinfo=datetime.timezone.utc) is True 😆.

I had it showing up in a pytest diff highlighting it as a reason for assertions failing but I guess it does that just based on the repr or something, the issue was another field.

[dvarrazzo]

They are two different python objects but they have the same semantic as timezone info objects:

>>> datetime.datetime(2000, 1, 1, tzinfo=datetime.timezone.utc) \
... == datetime.datetime(2000, 1, 1, 0, 0, tzinfo=zoneinfo.ZoneInfo(key='UTC'))
True

I wouldn't want to ensure the identity of the object we return.

(I see you got to the same conclusion of semantic equivalence).