Start testing Python 3.15 Alpha

[jonathan343]

Note

This PR is modeled after psf/requests#7195

Overview

This PR adds Python 3.15 canary coverage in CI and fixes deterministic gzip fixture failures in tests/unit/test_compress.py on 3.15 pre-release runtimes.

Background

1. We want early signal for Python 3.15 regressions.
2. CPython gh-91349 changed default gzip compression level from 9 to 6, which changed our byte-for-byte gzip fixture output.
3. In 3.15-dev jobs, some dependencies may be built from source (instead of wheels) and use Rust/PyO3. PyO3 can reject unknown/new CPython minors by default during pre-release windows.

Summary

1. Add 3.15-dev to CI matrices in:
   • .github/workflows/run-tests.yml
   • .github/workflows/run-crt-test.yml
2. Set PYO3_USE_ABI3_FORWARD_COMPATIBILITY=1 only for 3.15-dev jobs.
   • This is a CI canary workaround for prerelease interpreter builds. It allows PyO3 abi3 extension builds to proceed on 3.15-dev and can be removed when Start building for python 3.15 pre-release crate-py/rpds#210 is resolved.
3. Keep gzip fixtures byte-stable by pinning mtime=1 and compresslevel=9 in tests/unit/test_compress.py.
4. Updates the locked typing_extensions version to address AttributeError: module 'typing' has no attribute 'no_type_check_decorator' errors.

Testing

# Sent PyO3 forward compatibility env var
export  PYO3_USE_ABI3_FORWARD_COMPATIBILITY=1

# Create Python 3.15 virtual environment
uv venv -p 3.15
source .venv/bin/activate

# Install package and dev requirements
uv pip install -e .
uv pip install -r requirements-dev-lock.txt

# Run unit and functional tests
python -m pytest tests/unit tests/functional

# Run integration tests
python -m pytest tests/integration