Use select.poll instead of select.select to support fds higher than 1024

[nivye]

select.select has a known issue in which it throws a ValueError exception for file descriptors higher than 1024 (similar issue at https://bugs.python.org/issue31610)

This change does not introduce any API changes and does not introduce any new behavior.
It solely replaces select.select by select.poll to be able to support file descriptors larger than 1024.

Also added a test to demonstrate the fix - replacing the new select.poll code portion by the previous select.select line, would raise a ValueError when handling the OperationalError.

[codecov]

Codecov Report

Merging #668 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #668   +/-   ##
=======================================
  Coverage   92.18%   92.18%           
=======================================
  Files          10       10           
  Lines        1037     1037           
  Branches      121      121           
=======================================
  Hits          956      956           
  Misses         65       65           
  Partials       16       16           

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