Update client.py

Check for `None` instead of relying on the truth value of whatever the user happens to pass in for `unix_socket_path`. I think it is preferable to blow up when passed something like `0` rather than silently making a TCP connection.
Update client.py

put spaces around operator
Update client.py

PEP 8
Merge pull request #390 from hydrogen18/master

Fix issue 389
Merge remote-tracking branch 'upstream/master'
Override _checkpid() in SentinelConnectionPool
Merge pull request #396 from reclosedev/sentinel-pool

Override _checkpid() in SentinelConnectionPool (#395)
Merge remote-tracking branch 'upstream/master'