Update redis backend support

[Nagico]

feat:

• native django redis-py, django-redis backend based on url connection: add USER field for authentication
• native django redis-py, django-redis backend based on socket connection: add USER and PASSWORD field for authentication
• django-redis-cache backend: throw exception when USER field is set
• test_redis.py file: replace pytest.mark.skipif with pytest.fixture and mock, add conftest.py file which provides django_cache_url_dj3 and django_cache_url_dj4, two mocked modules as fixtures, for each test function.

fix:

• native django redis-py backend: covert the keys in the OPTIONS to lower case
• uniform redis backends name:

# native django redis-py backend (Unchanged): 
BUILTIN_DJANGO_BACKEND = 'django.core.cache.backends.redis.RedisCache' 

# django-redis backend (Add a new variable)
DJANGO_REDIS_BACKEND = 'django_redis.cache.RedisCache'

# django-redis-cache (Unchanged): 
DJANGO_REDIS_CACHE_LIB_KEY = 'redis-cache'
DJANGO_REDIS_CACHE_BACKEND = 'redis_cache.RedisCache'

# default redis backend, depend on django version (Replace DJANGO_REDIS_BACKEND with DEFAULT_REDIS_BACKEND)
DEFAULT_REDIS_BACKEND = DJANGO_REDIS_BACKEND if VERSION[0] < 4 else BUILTIN_DJANGO_BACKEND

docs:

• readme.rst file: update redis, rediss, hiredis usage about authentication and native redis backend

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More information about available schemes of different redis backends

• native redis backend in Django 4 and redis-py
• django-redis
• django-redis-cache