Update CI for GitHub Actions and run tests against newer Python/Django/Celery

[philipstarkey]

Fixes #86 #79

Rationale

Most of the current dependency versions tested against are unsupported, while newer versions are not tested. Also, the existing CI seems to be dead (at least the badge indicates it is).

Changes

• Dropped support for Python 3.7, Django 2.2, and Celery 4.
• Added CI tests for Python 3.10-3.12, Django 4.0-5.0, and Celery 5.2-5.3 (existing tests against earlier Python 3.8+, Django 3.0+, and Celery 5.0+ kept)
• Added a GitHub action workflow that runs the tox tests. This workflow will run when a PR is created/updated and when changes are pushed to the repository.
• Remove travis config
• Updated readme with the new requirements and changelog
• Bumped the major version number due to the breaking changes (dropping support for older dependencies, see first point above)
• Updated setup.py to reflect dependency support changes
• Add vscode config exclusion to .gitignore file