Bump minimum OS X deployment version to 10.15

[darbyjohnston]

Fixes #1892

Now that 2022 is our minimum VFX Reference Platform, we can bump the OS X deployment version to 10.15:
https://vfxplatform.com/platform_history.html

[darbyjohnston]

There seems to be an issue with the CI; the PR says the checks have passed, but the actions failed with the error: "Startup failure".

[meshula]

yay!

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.80%. Comparing base (c0e97b0) to head (13229ff).
⚠️ Report is 106 commits behind head on main.

❌ Your changes status has failed because you have indirect coverage changes. Learn more about Unexpected Coverage Changes and reasons for indirect coverage changes.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1893      +/-   ##
==========================================
+ Coverage   84.11%   84.80%   +0.68%     
==========================================
  Files         198      177      -21     
  Lines       22241    12789    -9452     
  Branches     4687     1191    -3496     
==========================================
- Hits        18709    10846    -7863     
+ Misses       2610     1760     -850     
+ Partials      922      183     -739     

Flag	Coverage Δ
py-unittests	84.80% <ø> (+0.68%)	⬆️

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see 136 files with indirect coverage changes

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