Django 4.0 Support

[johnthagen]

Closes #76

Closes #108

#104 instates the policy for supporting supported Django versions and thus this PR is compatible with that new policy.

• Officially support Django 2.2, 3.2, and 4.0, the versions of Django currently supported
• Do not require Django to be installed simply to build the source distribution for django-solo. This enables testing to work when only tox is installed (e.g. CI)

[johnthagen]

@lazybird Given that Django 4 is only a month away, I think it would be great if you were able to review this and #106, and we published a Django 4-compatible version of django-solo to PyPI before then.

[johnthagen]

@lazybird Another friendly ping to see if we can get this all released before Django 4.0 is released.

[Archmonger]

@johnthagen If this package wants to maintain compatibility with Django 1.6+, the deprecated imports will need to remain.

Otherwise, the description needs to be updated to show the new minimum Django version.

Also, the same changes need to take place in solo/templatetags/solo_tags.py

[johnthagen]

If this package wants to maintain compatibility with Django 1.6+, the deprecated imports will need to remain.

@Archmonger Minimum Django version is specified in #104. Ultimately this is @lazybird's call, but in that PR I recommend following what most Django packages do which is to only support the currently supported Django versions. Users on older versions of Django can pin to older versions of django-solo.

The intention was to merge #104 before this PR and I was hoping @lazybird would give an official blessing on that PR before proceeding.

[johnthagen]

@connor-makowski @frague59 Thank you for your contributions in your PRs. I've consolidated everything including automating testing into this PR.

Please review this PR.

[connor-makowski]

Confirming pip installing from the current branch (git+https://github.com/johnthagen/django-solo.git@4f8c586121cac4a08a9d05b52e8f7c23366a156d) is working for me in django 4.0. and in old projects in django 3.1.

Thanks @johnthagen

[johnthagen]

@connor-makowski Would you mind trying one more time? I added an explicit install_requires to setup.py so that pip can properly enforce dependency resolution. This should help notify users explicitly if they are trying to install django-solo on an unsupported Django version.