Django 4.0 Compatibility

[jamesvandyne]

This PR updates django-webmention to work with Django 4.0 or higher. It does this by:

• Upgrading the urls configuration to use re_path instead of the removed django.conf.urls.url.
• Passing in a get_response value to the the middleware test fixture.

In addition, I've also updated the setup.cfg to test/notate compatibility with Python 3.9 - 3.11 and Django 3.2 - 4.1.

[jamesvandyne]

I wonder if it wouldn't be worthwhile to remove support for older, unsupported versions of Django?

[daneah]

Thanks for the contribution @jamesvandyne! Django 3.2 and Python 3.7 are now the lowest supported versions, so I recommend we drop support for earlier versions in this same change set as well.

[daneah]

Additional notes:

• This will be a major version bump due to drop of support
• The README has some stale mentions of old versions and conditions about what to do if using them. Those will need to be brushed up.

[jamesvandyne]

@daneah I've dropped support for unsupported versions of Django in the setup.cfg and tweaked the readme. If you think the readme needs further tweaking, please feel free to commit to this branch / PR any changes.

[daneah]

I will try to get a release of this out shortly; this project doesn't currently follow my usual flow so I'll need to do a little archaeology. I'll hopefully be able to follow up on adhering to that flow soon after.