Draft: Add Python 3.15 to build matrix

[jminor]

Python 3.15 will be released soon (see https://blog.python.org/2026/09/python-3150-rc2/ ) so let's get ahead of things and verify that OTIO works with 3.15.

This PR builds on top of #2032

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.48%. Comparing base (b3ebf10) to head (1cd922d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2047   +/-   ##
=======================================
  Coverage   83.47%   83.48%           
=======================================
  Files         182      182           
  Lines       13517    13517           
  Branches     1254     1254           
=======================================
+ Hits        11283    11284    +1     
+ Misses       2061     2060    -1     
  Partials      173      173           

Flag	Coverage Δ
py-unittests	83.48% <ø> (+<0.01%)	⬆️

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