Requirements: downgrade redis to 3.5.3

It seems that new pip version complains about this error now and breaks the
installation process. We are pinning to the latest 3.x version of redis for now.

I think we can upgrade the django-redis-cache version once we are on Django 3.2.x

Author: humitos

requirements/pip.txt

@@ -31,7 +31,11 @@ pyyaml==5.4.1  # pyup: ignore
 Pygments==2.10.0
 
 # Basic tools
-redis==4.1.0
+
+# Downgrade redis to 3.5.3 (latest 3.x release) because pip raises this error:
+# django-redis-cache 2.1.3 depends on redis<4.0
+redis==3.5.3
+
 kombu==5.2.2
 celery==5.2.2