Refactor how asyncio.BlockingConnectionPool gets connection.

[kristjanvalur]

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Description of change

This is an update to previously merged pr #2997
We use inheritance to simplify the logic, the BlockingConnectionPool only needs to override get_available_connection()

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (5391c5f) 91.36% compared to head (b886731) 91.37%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2998   +/-   ##
=======================================
  Coverage   91.36%   91.37%           
=======================================
  Files         126      126           
  Lines       32617    32611    -6     
=======================================
- Hits        29801    29798    -3     
+ Misses       2816     2813    -3     

Files	Coverage Δ
redis/asyncio/connection.py	84.00% <100.00%> (+0.34%)	⬆️

... and 2 files with indirect coverage changes

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