[IP-6943] enhance redis-namespace to work with redis-py 4.5.x and Redis 5.0.x

[rbevers]

• switched CI to use Redis 5.0.x
• switched redis-py to use 4.5.x
• cleaned out references to redis._compat

[rbevers]

A bunch of test failures that need going through, one-by-one @codergolem. But I made some progress.