Fixture for Async Client

[ResleyR]

With Django 3.1 it would be useful to have an async_client fixture similar to the existing client fixture.
The following code works but would be nice if it was part the plugin itself

@pytest.fixture()
def async_client():
    """A Django async test client instance."""
    skip_if_no_django()

    if get_django_version() < (3, 1):
        pytest.skip("no Django settings")
    from django.test.client import AsyncClient

    return AsyncClient()

This allows for testing async views when used along with pytest-asyncio with the alternative being using the django TestCase instead.

[bluetech]

Fixed in #865, will be in 4.1 (along with async_rf).

The fixture as written doesn't do the automatic pytest.skip on django<3.1. Personally not sure automatic skip is a good idea...