Add support for Django 3.2

[con-cat]

• Configure default auto field for waffle and test_app - Django 3.2 adds the ability to customise the type of auto-created primary key fields.
• Add Django 3.2 to tox.ini and setup.py

Addresses #395 Fixes #396

[clintonb]

Since this feature pertains to Django 3.2, please implement #395, and update tox.ini and setup.py.

[con-cat]

Since this feature pertains to Django 3.2, please implement #395, and update tox.ini and setup.py.

@clintonb I've done that, and also updated run.sh to fix RemovedInDjango40Warning: django-admin.py is deprecated in favor of django-admin.

[con-cat]

I've removed the main branch of Django from the tox.ini envlist because of a few things that are incompatible with Django 4.0 causing tests to fail - I think it would make sense to address those separately.

Specifically, I tried replacing ugettext_lazy with gettext_lazy to address the error in the CI run above, but then I received the below error about a migration changing NullBooleanField to BooleanField, which is funny because NullBooleanField should still be ok in historical migrations.

ValueError: Cannot alter field waffle.Flag.everyone into waffle.Flag.everyone - they do not properly define db_type (are you using a badly-written custom field?)