[Backport] Add lock to Pubsub.execute_command to ensure only one connection is created

[dlunch]

Pull Request check-list

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• Does $ tox pass with this change (including linting)?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?
• Was the change added to CHANGES file?

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Description of change

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2708435) 91.49% compared to head (328f79e) 91.50%.

Additional details and impacted files

@@           Coverage Diff           @@
##              4.4      #22   +/-   ##
=======================================
  Coverage   91.49%   91.50%           
=======================================
  Files         116      116           
  Lines       30320    30336   +16     
=======================================
+ Hits        27742    27759   +17     
+ Misses       2578     2577    -1     

Files	Coverage Δ
redis/client.py	87.22% <100.00%> (+0.14%)	⬆️
tests/test_pubsub.py	76.62% <100.00%> (+0.41%)	⬆️

... and 2 files with indirect coverage changes

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