Support current Django versions

This MR fixes issue #6 (closed) by updating django-ucamwebauth to support Django 3.2 (LTS) and 4.0 (the currently-supported versions). Django 2.2 is out of support on April 1st, so I've pre-emptively dropped support for it, and bumped the minimum-supported Python version to 3.7, which is the earliest version supported by Django 3.2. I've bumped the package major version to 2.0.0 to reflect these breaking changes (not sure if you're using semver for your versions). (I figure that anyone still using Django 2.2 can continue using the existing 1.x version until they upgrade, and increasing the minimum Python/Django version should make the project easier to maintain going forward.)

If that's too aggressive, the package does work with Django 2.2 as-is, it's only the version constraints that exclude 2.2.

I also added a Visual Studio Code devcontainer config, to allow the project to be worked on in a container dev environment in VS Code. (I find these pretty good for making it easy to jump between projects without needing to tend to local environment setup each time.)

Finally, I also added a tox config which tests the project with the same matrix of Python and Django versions as the CI job, but locally. It also tests the built package, which should give some added confidence that a release is working. The devcontainer has all the required Python versions installed, so just running $ tox (or $ tox --parallel) in the devcontainer should work without any manual setup.