Release candidate for django-csp 3.8

It's been more than a year since the last release and the project needed some refreshing before we can move forward with it.

This release aims to be functionally equivalent to 3.7, but with formal support for more modern Django and Python versions, all the way up to Django 5 on Python 3.12

Please see https://github.com/mozilla/django-csp/blob/3.8rc/CHANGES for a short summary of changes.

Feedback and bug reports are very welcome.

What's Changed (based on commit history)

Update installation.rst by @Jesus805 in #163
Fix 164 migrate ci by @g-k in #165
Update test configuration to cover up to Py3.9 and Django 3.2 by @mkoistinen in #172
Remove deprecation warning for child-src by @rik in #154
add project_urls to setup.py by @pawl in #171
Drop old Django and Python versions by @g-k in #175
rename default branch by @g-k in #176
Update CI badge to CircleCI by @g-k in #177
fix unwrap script re by @g-k in #178
Tweak configuration docs by @jaap3 in #146
docs: add note about nonce value visibility by @g-k in #180
GH-182 Update docs to clarify when nonce will not be added to headers by @DylanYoung in #185
Remove outdated docs reference to MIDDLEWARE_CLASSES by @mlazar-endear in #193
updating csp_replace decorator doc by @chestnutcone in #183
Wrap the test install with quotes. by @tim-schilling in #200
Reawaken development by @stevejalim in #204
Add readthedocs config and slightly update Sphinx config by @stevejalim in #205
Ensure docs building has access to django_csp itself by @stevejalim in #206
Add Sphinx RTD theme by @stevejalim in #207
Improve themeing in RTD by @stevejalim in #208
Update settings documentation to move deprecated-within-csp settings to their own section, at the bottom by @stevejalim in #210
MiddlewareMixin is always present in django>=3.2 by @asottile-sentry in #211
Bring codebase up to modern Python using pyupgrade by @stevejalim in #213
Update GH actions helpers to use Node 20-based versions by @stevejalim in #214
Prepare for 3.8rc release by @stevejalim in #215