Support all cache backends of Django >=3.2

[tneuct]

When upgrading to Django 3.2, the django-prometheus package caused some issues because it forces the user to use the PyLibMCCache without making the implicit requirement of pylibmc transparent to the user or directly requiring the package (introduced by #296). Additionally, the deprecation warning is hidden from the user.

Django 3.2 gives the user the option to keep on using the deprecated backend, and also adds another cache backend than the pylibmc-based one.

This merge request adds support for all cache backends of Django 3.2 and lets the user choose between them according to the official Django documentation.

This closes issue #328

[asherf]

thanks. sorry for not reviewing sooner.
I'll try to land this in a release in the next week or so.

[mounirmesselmeni]

Any plan for a release soon? I also need this but it's not in the current pypi release.
Thank you!