Fix: automatically close connection pool for async Sentinel

[kristjanvalur]

Pull Request check-list

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Description of change

A Redis object returned by redis.asyncio.sentinel.Sentinel() has its own ConnectionPool, but this is not closed automatically when the Redis object goes out of scope.

By setting the auto_close_connection_pool Redis attribute, it is ensured that connections are closed properly.

This fixes another issue raised in #2831

Please provide a description of the change here.

[codecov-commenter]

Codecov Report

Patch coverage: 16.00% and project coverage change: -24.85% ⚠️

Comparison is base (6968431) 91.27% compared to head (0d17f06) 66.43%.

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Additional details and impacted files

@@             Coverage Diff             @@
##           master    #2900       +/-   ##
===========================================
- Coverage   91.27%   66.43%   -24.85%     
===========================================
  Files         126      126               
  Lines       32368    32391       +23     
===========================================
- Hits        29545    21518     -8027     
- Misses       2823    10873     +8050     

Files Changed	Coverage Δ
redis/asyncio/sentinel.py	29.88% <0.00%> (-56.87%)	⬇️
tests/test_asyncio/test_sentinel.py	35.22% <26.66%> (-64.09%)	⬇️

... and 80 files with indirect coverage changes

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