Various cleanups and bug fixes

[arnimarj]

In an effort to fix some lingering issues, we've:

1. removed code which we aren't using, including the shard/sentinel/unix/monitor/subscriber/watch connection pools.
2. explicit pipelining and transaction support
3. removed all but python 3.7 CI support
4. made hiredis mandatory

Plus, some issues were fixed, including:

1. The deferred object in RedisFactory was fired when the factory was lazy, resulting in an unhandled deferred on GC events. Easy to test by forcing trial to GC between test runs
2. Fixed a silly bug in notifying waiters and peekers on Peekable queue, which would result in peekers not being notified on errors
3. Fixed a silly typo for raiseExcpetion

[arnimarj]

Oops, meant for this to we a PR for my fork