Drop Django 3.2 and 4.1 from the build matrix, add Django master

[jieter]

Some much needed maintenance in various configuration files for CI/test running bringing the Django versions we support in line with those currently supported by Django:

• Drop support for django 3.2, 4.1 (https://www.djangoproject.com/download/#supported-versions)
• Dropped support for Postgres 12 (https://docs.djangoproject.com/en/dev/releases/5.1/#dropped-support-for-postgresql-12)
• Updated documentation for supported versions
• Updated trove classifiers
• Build docs in CI
• Bring tox.ini in line with the current supported versions

[danielvdp]

Changelog?

[jieter]

Changelog?

The changelog is authored while making the release, not while making changes through pull requests.

[jieter]

Thanks!