Add Django 4.0 support, drop official Django 3.1 support

[amureki]

Official Django 3.1 support stopped a couple of weeks ago.
I think, it is time to drop it from test matrix.
Having 2.2 and 3.2 LTS releases implies that 3.1 would work fine.
I also dropped main Django branch from test matrix as I don't see much value for us here at this moment.

[amureki]

There is an issue with multidatabase tests:

 _ ERROR at setup of TestBakerAllowsSaveParameters.test_allow_user_to_specify_database_via_save_kwargs_for_retro_compatibility _
  .tox/py38-django40-postgresql/lib/python3.8/site-packages/django/test/testcases.py:1208: in setUpClass
      cls.cls_atomics = cls._enter_atomics()
  .tox/py38-django40-postgresql/lib/python3.8/site-packages/django/test/testcases.py:1185: in _enter_atomics
      atomics[db_name].__enter__()
  .tox/py38-django40-postgresql/lib/python3.8/site-packages/django/db/transaction.py:189: in __enter__
      if not connection.get_autocommit():
  .tox/py38-django40-postgresql/lib/python3.8/site-packages/django/db/backends/base/base.py:400: in get_autocommit
      self.ensure_connection()
  .tox/py38-django40-postgresql/lib/python3.8/site-packages/django/utils/asyncio.py:25: in inner
      return func(*args, **kwargs)
  .tox/py38-django40-postgresql/lib/python3.8/site-packages/django/db/backends/base/base.py:230: in ensure_connection
      self.connect()
  .tox/py38-django40-postgresql/lib/python3.8/site-packages/django/test/testcases.py:150: in __call__
      raise AssertionError(self.message)
  E   AssertionError: Database connections to 'extra' are not allowed in this test. Add 'extra' to pytest_django.fixtures._django_db_fixture_helper.<locals>.PytestDjangoTestCase.databases to ensure proper test isolation and silence this failure.

Related issues:

• Mutl databases (multi db) support pytest-dev/pytest-django#924
• https://code.djangoproject.com/ticket/33153

[berinhard]

Thanks for setting this up @amureki. Once Django 4.0 is ready, we'll be able to fix this, merge and release a new version.

[amureki]

@berinhard I am not sure that final Django release will solve this issue. The problem is in multi-db setup here and our mix of pytest (which has not 100% support of such thing yet) and usual Django test suite. I need a bit more dedication to attempt to solve it.

But any help appreciated. :)

[berinhard]

Got it @amureki! I'll try to investigate this issue before django 4.0 release, if possible

[berinhard]

Hey @amureki the fixes on CI also solved the issue we're having with this PR. Now it's actually failing due to the tests on django 4.0 =)

[amureki]

@berinhard it is finally green. 🤩