Add core redis compat if django 4+

[glasnt]

Resolves #356

Now django-environ supports Django 4.0, this feature can be added: use the core redis library by default if running Django 4.0

(Would appreciate input how to best test this)

[sergeyklay]

@glasnt Could you please send this PR to develop branch?

[glasnt]

@sergeyklay that should be fixed now. My apologies!

[coveralls]

Coverage increased (+0.06%) to 91.667% when pulling 7ee07ab on glasnt:feat/redis-django into 27fcdc3 on joke2k:develop.

[sergeyklay]

@glasnt Tests failed. See logs

2022-06-16T07:50:12.1170507Z     def test_cache_parsing(url, backend, location):
2022-06-16T07:50:12.1170795Z         url = Env.cache_url_config(url)
2022-06-16T07:50:12.1171022Z     
2022-06-16T07:50:12.1171332Z >       assert url['BACKEND'] == backend
2022-06-16T07:50:12.1171863Z E       AssertionError: assert 'django.core....is.RedisCache' == 'django_redis...he.RedisCache'
2022-06-16T07:50:12.1172533Z E         - django_redis.cache.RedisCache
2022-06-16T07:50:12.1172876Z E         + django.core.cache.backends.redis.RedisCache

Could you adjust tests? Also note I changed a bit your patch.

[sergeyklay]

Thank you for the patch, and for helping make this project better!

[glasnt]

Thank you @sergeyklay for your help on this one! I was going to address your comments this morning but you've already done it! ✨

[sergeyklay]

Yes, I wanted to merge it ASAP. Thank you very much for your input!