This change moves to the new TOML-based format for tox configuration,

and makes several fixes and improvements:

Fixed typos in postgres environments.
Fixed envlist to include db-specific environments for specific

  python/django combinations (before this was listed in the comments but

  didn't actually work).
Made it so our test setup will bail in the event that we're running

  with a db+django version that is not supported (since I still haven't

  figured out how to make this work with the tox config). This makes it

  so running dbtests-mysql5_7 won't fail with lots of errors when it

  tries to run against all the django4.2+ envs.
Tweaked Python versions for Django 4.x

Ran tests for various environments.