fix(recipe): fix deprecation warning from threading.Event

[stefreak]

Fix deprecation warning emitted when calling threading.Event.isSet

Why is this needed?

Reasoning:

• is_set is supported since Python2.6 (https://docs.python.org/2.7/library/threading.html)
• Starting from Python3.5 docs the isSet spelling is not even mentioned anymore (https://docs.python.org/3.5/library/threading.html)
• Python3.10 will give a deprecation warning when the isSet spelling is used (https://docs.python.org/3.10/library/threading.html)

Proposed Changes

• just use is_set over isSet

Does this PR introduce any breaking change?

• only for users of python version below 2.6; python2.5 is not supported according to setup.py

[stefreak]

I'll be glad to rebase once #652 is merged. Just let me know if I overlook it :)

[stefreak]

@ceache sure, thanks a lot for the feedback! I have changed the commit message accordingly.

[stefreak]

I rebased with master, hoping that it would run the github actions for my PR, but it didn't; sorry for dismissing your reviews by doing that @ceache and @StephenSorriaux

[StephenSorriaux]

Sorry about that, that is actually on me. I will fix it.

[StephenSorriaux]

@stefreak Can you please rebase your PR with the current master that fixes the CI part? Thank you.

[stefreak]

@StephenSorriaux done :)

[codecov-commenter]

Codecov Report

Merging #657 (569c89c) into master (1ea097d) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #657   +/-   ##
=======================================
  Coverage   94.20%   94.20%           
=======================================
  Files          56       56           
  Lines        8246     8246           
=======================================
  Hits         7768     7768           
  Misses        478      478           

Impacted Files	Coverage Δ
kazoo/recipe/lock.py	97.95% <100.00%> (ø)
kazoo/recipe/queue.py	97.45% <100.00%> (ø)
kazoo/testing/harness.py	96.66% <100.00%> (ø)
kazoo/tests/test_lock.py	98.89% <100.00%> (ø)
kazoo/tests/test_connection.py	97.70% <0.00%> (-0.39%)	:arrow_down:
kazoo/handlers/threading.py	91.94% <0.00%> (+0.67%)	:arrow_up:

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