Add support for Python 3.14

[hgreebe]

Description of changes

• Add support for Python 3.14 to the ParallelCluster CLI.
  1. Replace deprecated asyncio.get_event_loop() calls with a new utility function.
     In Python 3.14, get_event_loop() raises RuntimeError when no event loop is running instead of implicitly creating one.
     This is documented in Python official doc: https://docs.python.org/3.14/whatsnew/3.14.html#asyncio
  2. Add explicit support for Python 3.14.
  3. Enable unit tests for Python 3.14

Tests

• Manually verified that the CLI works with Python 3.13 and 3.14
• Unit tests

References

• Based off of: Add support for Python 3.14 #7149

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.07%. Comparing base (abee849) to head (4f0b2d0).
⚠️ Report is 6 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7574      +/-   ##
===========================================
+ Coverage    89.94%   90.07%   +0.13%     
===========================================
  Files          180      180              
  Lines        16255    16400     +145     
===========================================
+ Hits         14621    14773     +152     
+ Misses        1634     1627       -7     

Flag	Coverage Δ
unittests	90.07% <100.00%> (+0.13%)	⬆️

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