Add support for Python 3.15

[kytta]

Description of changes: The first release candidate for Python 3.15 has been out for quite some time. I think it's time to officially declare support for it (and add it to the testing matrix). I have ran tests on both 3.15 and 3.15t locally, and both suites pass. I had to bump rpds-py to a slightly newer version that supports freethreaded and modern Python; otherwise the tests would fail.

See also:

• Add support for Python 3.15 s3transfer#416
• Add support for Python 3.15 boto3#4833

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.47%. Comparing base (ee748e4) to head (bad95a7).
⚠️ Report is 482 commits behind head on develop.
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Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3785      +/-   ##
===========================================
- Coverage    92.59%   92.47%   -0.13%     
===========================================
  Files           68       68              
  Lines        15634    15859     +225     
===========================================
+ Hits         14477    14666     +189     
- Misses        1157     1193      +36     

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