text: Title: pytest==6.1.0: processing filter warnings is done before django.setup()
  Body: Lauching pytest results in the following error:
  <details>
  <summary><tt>pytest test_foo.py</tt> click to expand</summary>
  <pre>
  Traceback (most recent call last):
    File "/home/presto/.virtualenvs/presto/bin/pytest", line 8, in <module>
      sys.exit(console_main())
    File "/home/presto/.virtualenvs/presto/lib/python3.8/site-packages/_pytest/config/__init__.py", line 187, in console_main
      code = main()
    File "/home/presto/.virtualenvs/presto/lib/python3.8/site-packages/_pytest/config/__init__.py", line 143, in main
      config = _prepareconfig(args, plugins)
    File "/home/presto/.virtualenvs/presto/lib/python3.8/site-packages/_pytest/config/__init__.py", line 318, in _prepareconfig
      config = pluginmanager.hook.pytest_cmdline_parse(
    File "/home/presto/.virtualenvs/presto/lib/python3.8/site-packages/pluggy/hooks.py", line 286, in __call__
      return self._hookexec(self, self.get_hookimpls(), kwargs)
    File "/home/presto/.virtualenvs/presto/lib/python3.8/site-packages/pluggy/manager.py", line 93, in _hookexec
      return self._inner_hookexec(hook, methods, kwargs)
    File "/home/presto/.virtualenvs/presto/lib/python3.8/site-packages/pluggy/manager.py", line 84, in <lambda>
      self._inner_hookexec = lambda hook, methods, kwargs: hook.multicall(
    File "/home/presto/.virtualenvs/presto/lib/python3.8/site-packages/pluggy/callers.py", line 203, in _multicall
      gen.send(outcome)
    File "/home/presto/.virtualenvs/presto/lib/python3.8/site-packages/_pytest/helpconfig.py", line 100, in pytest_cmdline_parse
      config = outcome.get_result()  # type: Config
    File "/home/presto/.virtualenvs/presto/lib/python3.8/site-packages/pluggy/callers.py", line 80, in get_result
      raise ex[1].with_traceback(ex[2])
    File "/home/presto/.virtualenvs/presto/lib/python3.8/site-packages/pluggy/callers.py", line 187, in _multicall
      res = hook_impl.function(*args)
    File "/home/presto/.virtualenvs/presto/lib/python3.8/site-packages/_pytest/config/__init__.py", line 1003, in pytest_cmdline_parse
      self.parse(args)
    File "/home/presto/.virtualenvs/presto/lib/python3.8/site-packages/_pytest/config/__init__.py", line 1280, in parse
      self._preparse(args, addopts=addopts)
    File "/home/presto/.virtualenvs/presto/lib/python3.8/site-packages/_pytest/config/__init__.py", line 1186, in _preparse
      self.hook.pytest_load_initial_conftests(
    File "/home/presto/.virtualenvs/presto/lib/python3.8/site-packages/pluggy/hooks.py", line 286, in __call__
      return self._hookexec(self, self.get_hookimpls(), kwargs)
    File "/home/presto/.virtualenvs/presto/lib/python3.8/site-packages/pluggy/manager.py", line 93, in _hookexec
      return self._inner_hookexec(hook, methods, kwargs)
    File "/home/presto/.virtualenvs/presto/lib/python3.8/site-packages/pluggy/manager.py", line 84, in <lambda>
      self._inner_hookexec = lambda hook, methods, kwargs: hook.multicall(
    File "/home/presto/.virtualenvs/presto/lib/python3.8/site-packages/pluggy/callers.py", line 208, in _multicall
      return outcome.get_result()
    File "/home/presto/.virtualenvs/presto/lib/python3.8/site-packages/pluggy/callers.py", line 80, in get_result
      raise ex[1].with_traceback(ex[2])
    File "/home/presto/.virtualenvs/presto/lib/python3.8/site-packages/pluggy/callers.py", line 182, in _multicall
      next(gen)  # first yield
    File "/home/presto/.virtualenvs/presto/lib/python3.8/site-packages/_pytest/warnings.py", line 136, in pytest_load_initial_conftests
      with catch_warnings_for_item(
    File "/usr/lib/python3.8/contextlib.py", line 113, in __enter__
      return next(self.gen)
    File "/home/presto/.virtualenvs/presto/lib/python3.8/site-packages/_pytest/warnings.py", line 52, in catch_warnings_for_item
      apply_warning_filters(config_filters, cmdline_filters)
    File "/home/presto/.virtualenvs/presto/lib/python3.8/site-packages/_pytest/config/__init__.py", line 1602, in apply_warning_filters
      warnings.filterwarnings(*parse_warning_filter(arg, escape=False))
    File "/home/presto/.virtualenvs/presto/lib/python3.8/site-packages/_pytest/config/__init__.py", line 1576, in parse_warning_filter
      category = warnings._getcategory(
    File "/usr/lib/python3.8/warnings.py", line 260, in _getcategory
      m = __import__(module, None, None, [klass])
    File "/home/presto/Projects/sandbox/pytest_bug_warning/bunny/models.py", line 10, in <module>
      class Bunny(models.Model):
    File "/home/presto/.virtualenvs/presto/lib/python3.8/site-packages/django/db/models/base.py", line 108, in __new__
      app_config = apps.get_containing_app_config(module)
    File "/home/presto/.virtualenvs/presto/lib/python3.8/site-packages/django/apps/registry.py", line 253, in get_containing_app_config
      self.check_apps_ready()
    File "/home/presto/.virtualenvs/presto/lib/python3.8/site-packages/django/apps/registry.py", line 136, in check_apps_ready
      raise AppRegistryNotReady("Apps aren't loaded yet.")
  django.core.exceptions.AppRegistryNotReady: Apps aren't loaded yet.
  </pre>
  </details>
  
  The project is straighforward.
  
  We created the following Warning and Model:
  ``` python
  from django.db import models
  
  # Create your models here.
  
  
  class CarrotWarning(RuntimeWarning):
      pass
  
  
  class Bunny(models.Model):
      pass
  ```
  
  And use the following `setup.cfg`:
  ```
  [tool:pytest]
  filterwarnings =
      ignore::bunny.models.CarrotWarning
  ```
  
  The bug was introduced by `pytest==6.1.0`, it worked fine with `pytest==6.0.2`.
  
  We get that it seems to be introduced by pytest itself, but as its django-related, we assume that the fix will be in `pytest-django`.
  
  We would guess from the changelog that it's linked to this issue : https://github.com/pytest-dev/pytest/issues/6681
  