Update Redis dependency >4.0

[salomon-smekecohen]

The redis version pin in baseplate includes the <4.0.0 clause. Redis has a vulnerability in 3.x versions.

See the following for a quick scan of potential incompatibilities from the changelog:

• BACKWARDS INCOMPATIBLE: All values within Redis URLs are unquoted via
  urllib.parse.unquote. Prior versions of redis-py supported this by
  specifying the decode_components flag to the from_url functions.
  This is now done by default and cannot be disabled. #589
• POTENTIALLY INCOMPATIBLE: Redis commands were moved into a mixin
  (see commands.py). Anyone importing redis.client to access commands
  directly should import redis.commands. #1534, #1550