Enable middleware decorator to work in newer versions of Django

[daneah]

Resolves #6

django-webmention provides both an old-style and a new-style middleware, but the provided decorator wrapped only the old-style middleware. By inheriting from django.utils.deprecation.MiddlewareMixin, the middleware and the decorator become compatible with both styles.

Leaving the standalone new-style middleware in for backward compatibility as well. Dropping support for Python 3.5 because Django 3.0+ no longer supports it.

Adding automation for releasing to GitHub releases and PyPI as I hadn't done that prior.

[daneah]

@philgyford have a look! Turned out to be similar to what we discussed.