re-add support for redis-py==3 and add CI check

[nsaje]

https://github.com/closeio/tasktiger/pull/275 broke support for redis-py<4

• fixed the problematic import
• bumped the required redis-py version from >=3 to >=3.3.0 - TT didn't work with less for a long while now probably
• added redis-py into the CI matrix so we actually test if tests pass with lower redis-py versions