include tests for higher versions of Python and Django

[newearthmartin]

I just submitted a PR #391 and the tests cover python 3.7 to 3.10 and Django 2.2 to 3.2.

Can Python versions 3.11 and 3.12 and Django versions 4 and 5 be included?

[newearthmartin]

Also this seems to be inaccurate in relation to the actual tests:

[sebastian-muthwill]

I just tried to get this package working since I would love to include it in one of my projects. Hence the dependencies are quite outdated.
I could try to bring together a few of the PR's to support new versions of python and django as already someone did in #389

What do you think about dropping support for Python <3.8 so #391 would not be necessary. In general also 3.8 is already EOL.

Currently I just enrolled as a member of the Jazzband and tbh I'm not really sure if I'm allowed to do any changes / merges in this repo (even if I have the rights to do so :-))

[sebastian-muthwill]

@newearthmartin I created a PR (#394) with necessary changes to make the repo up to date and could need a second pair of eyes. Need to update the documentation though.

[newearthmartin]

@sebastian-muthwill Ok let me take a look

[newearthmartin]

@sebastian-muthwill I sent you some comments as code review. Main question is: Why are we dropping so many Django versions?

[newearthmartin]

Why are we dropping so many Django versions?

Expanding on this: These versions could be EOL for official Django support, but if there is no technical conflict it's not a reason for us to drop them. There may be many projects using django-newsletter with older Django versions.