Start testing Python 3.15 Alpha

[jonathan343]

Note

This is the S3transfer version of boto/botocore#3628

Overview

This PR adds Python 3.15 canary coverage in CI to give us early signal for Python 3.15 regressions.

Testing

# 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

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[ubaskota]

LGTM