Checking if pipeline is empty prior to executing per discussion in #362
returning empty list when empty pipeline run per discussion in #362
changelog
fix zrevrange parameter name where there is a semantic error.
check : http://www.redis.io/commands/zrevrange
Merge pull request #379 from xiaost-bytedance/redis-py-fix

fix zrevrange parameter name where there is a semantic error.
Improved sentinel commands support
Add redis.sentinel module
Add docstrings for Sentinel class
README.rst: add Sentinel support section
Rework slave selection logic
Allow to specify user's own connection pool class
SentinelConnectionPool: add check_connection flag
PEP8 fix