Bugfix: Ensure blacklist expires_at is set in a fully UTC context

[jgruppuso]

This was found in a context where UTC was not the Django timezone, but rather Asia/Tokyo. I have added a test which replicates the issue.

Before:

(Pdb) iat
1685329261
(Pdb) expires_at_unix_time
1685331061.0
(Pdb) datetime.utcfromtimestamp(iat)
datetime.datetime(2023, 5, 29, 3, 1, 1)
(Pdb) datetime.utcfromtimestamp(expires_at_unix_time)
datetime.datetime(2023, 5, 29, 3, 31, 1)
(Pdb) make_aware(datetime.utcfromtimestamp(expires_at_unix_time))
datetime.datetime(2023, 5, 29, 3, 31, 1, tzinfo=zoneinfo.ZoneInfo(key='Asia/Tokyo'))

iat (UTC)
expires_at_unix_time = iat + 30mins (UTC)
expires_at = iat + 30mins (Asia/Tokyo) → this means the token is created already expired!

After:

(Pdb) iat
1685335304
(Pdb) expires_at_unix_time
1685940104.0
(Pdb) datetime.datetime.utcfromtimestamp(iat)
datetime.datetime(2023, 5, 29, 4, 41, 44)
(Pdb) datetime.datetime.utcfromtimestamp(expires_at_unix_time)
datetime.datetime(2023, 5, 29, 5, 11, 44)
(Pdb) make_aware(datetime.datetime.utcfromtimestamp(expires_at_unix_time),timezone=datetime.timezone.utc)
datetime.datetime(2023, 5, 29, 5, 11, 44, tzinfo=datetime.timezone.utc)

iat (UTC)
expires_at_unix_time = iat + 30mins (UTC)
expires_at = iat + 30mins (UTC)