feat: add support for Python 3.15

[vdusek]

Adds Python 3.15 to the lint, type check and unit test matrices, uploads unit test coverage from 3.15, and adds the 3.15 classifier. Integration tests run on 3.11 and 3.15. Stacked on #1128, which bumps impit to the first release with 3.15 wheels.

datamodel-code-generator fails to import on 3.15, so test_model_generation_pipeline is skipped there until datamodel-code-generator/datamodel-code-generator#4609 ships. Removing the skip is tracked in #1127.

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.46%. Comparing base (fcff6fa) to head (acc8c07).

Additional details and impacted files

@@               Coverage Diff                @@
##           deps/impit-0.15    #1122   +/-   ##
================================================
  Coverage            95.46%   95.46%           
================================================
  Files                   61       61           
  Lines                 5935     5935           
================================================
  Hits                  5666     5666           
  Misses                 269      269           

Flag	Coverage Δ
integration	90.78% <ø> (-0.17%)	⬇️
unit	88.87% <ø> (ø)

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[Mantisus]

LGTM

[apify-approval-mirror]

Mirroring approval from @Mantisus.