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. E2E tests stay on 3.14 because there's no apify/actor-python:3.15 base image yet.

The 3.15 jobs fail at uv sync for now. impit 0.14.1 ships no 3.15 wheels, and building it from the sdist fails because the reqwest_unstable cfg from impit's .cargo/config.toml isn't applied. This stays a draft until impit releases 3.15 wheels.

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 31.73%. Comparing base (eb40f55) to head (70c7f52).
⚠️ Report is 1 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (eb40f55) and HEAD (70c7f52). Click for more details.

HEAD has 2 uploads less than BASE

Flag	BASE (eb40f55)	HEAD (70c7f52)
unit	1	0
integration	1	0

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1179       +/-   ##
===========================================
- Coverage   93.02%   31.73%   -61.30%     
===========================================
  Files          54       54               
  Lines        3671     3671               
===========================================
- Hits         3415     1165     -2250     
- Misses        256     2506     +2250     

Flag	Coverage Δ
e2e	31.73% <ø> (ø)
integration	?
unit	?

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