Fix handling of CancelledError by async client

[jajce]

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Description of change

This PR is supposed to fix the issue #2593 .
The problem occurs when invoking cancel() on a task which was executing any redis command. Invoking cancel() resulted in interruption of StreamReader readline() method by the CancelledError. When it happened it resulted in not reading the response from StreamReader after command execution. It caused all future responses to be mixed up between commands.

For example if we canceled a ping() command and executed exists('foo') command afterwards StreamReader reads previously not read b"+PONG\r\n" instead of b":0\r\n" which results in error when trying to parse raw data and cast it to the correct type. (This example is implemented as a working test)

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.03 ⚠️

Comparison is base (6c708c2) 92.31% compared to head (3390dfc) 92.28%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2607      +/-   ##
==========================================
- Coverage   92.31%   92.28%   -0.03%     
==========================================
  Files         115      115              
  Lines       29744    29772      +28     
==========================================
+ Hits        27459    27476      +17     
- Misses       2285     2296      +11     

Impacted Files	Coverage Δ
redis/asyncio/client.py	93.31% <100.00%> (+0.03%)	⬆️
tests/test_asyncio/test_connection.py	98.35% <100.00%> (+0.26%)	⬆️

... and 4 files with indirect coverage changes

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[dvora-h]

#2593 already fixed in #2695