Updates Test Configuration, Narrows Dependency Declaration

[iMerica]

Summary

This change narrows the supported Django and Django-All-Auth versions, as many tests are failing in various combinations of the two. Note: the tests are failing due to upstream dependencies, not new regressions in this package.

As a matter of caution, I've bumped the major version to signify the impact of the change.

Changes

• Moves Circle CI build config to use Tox to test all combinations of supported Python and Django Versions in CI.
• Adds Python 3.9 tests to tox configuration
• Pins specific versions of test dependencies.
• Narrows extras require of all-auth to django-allauth>=0.40.0,<0.43.0
• Updates README
• Bumps Major Version