Merge pull request #4158 from django-oscar/dependabot/pip/redis-gte-3.5-and-lt-5.1

Update redis requirement from <4.6,>=3.5 to >=3.5,<5.1

requirements.txt

 django-environ>=0.4,<0.11
 django-redis>=4.12,<5.3
 pysolr>=3.9,<3.10
-redis>=3.5,<4.6
+redis>=3.5,<5.1
 requests>=2.25,<3
 uWSGI>=2.0.19
 whitenoise>=5.2,<6.5