Remove enum34, convert to Pytest-style, add more supported targets

[hrichards]

From changelog:

3.1.0

• No functional changes: added support targets and refactored tests and dependencies.

Major

• Support Django 3.2 and 4.2.
• Add support for Python 3.8 and 3.9.
• Cope with psycopg2 bug: only 2.8 is supported through Django 2.2, but 2.9 is supported for higher Django versions.
• Parameterize test suite on USE_TZ to confirm Capone works for both True and False.
• Remove unneeded dependency enum34.
• Refactor to use django.utils.timezone.

Minor

• Convert tests to Pytest style.
• Default in tests to USE_TZ == True.
• Remove remaining Python 2 compatibility shims.
• Unpin lots of test dependencies.