Django4 Compatibility

[fleur]

This PR:

• fixes most of the flake8 errors & ignores line too long & line break before and after binary operator
• expands tox.ini to handle django up to 4.1 & python up to 3.10
• 3 commits to make django-mailbox compatible with django4
• revamps tox.ini to add github ci
• fixes first 3 badges and test status link at bottom (please note the build badge will not work until a github ci run has completed)
• fixed a couple deprecation messages seen in tox output

Since the build badge didn't link to anything current, I assumed the travis-ci integration was defunct. I realized after I did the github integration work that there was a PR in 2020 to fix it. Please let me know if you feel I should take out the github ci integration.

And from the list in #249:

• updates classifiers in setup.py
• updates CI to tests on Django 4.0, as mentioned above
• updates docs to mark Django 4.0 compatibility
• verifies if there is any pending migrations (answer: no)
• please note does NOT update release note to mark Django 4.0
• fixes any failing tests after CI update, as mentioned above

Also updated docs to remove travis-ci mentions.