Adds support for Django 5 and Moves to Github Actions for core CI/CD

[iMerica]

Changes

• Deprecates Python 3.6 and 3.7
• Integrates commits from PR https://github.com/iMerica/dj-rest-auth/pull/610
• Switches to Github Actions from Circle CI
• Adds flake8 linting step
• Corrects style to appease linter

[theognis1002]

when can this get merged in @iMerica ? thanks in advance 🙏