Bump Python versions: drop 3.9 (EOL), add 3.15

[Copilot]

Python 3.9 is EOL; drop it and add Python 3.15 as the new tracked pre-release.

Changes

• pyproject.toml: requires-python bumped from >=3.9 to >=3.10
• .github/workflows/main.yml:
  • Promote 3.10 from ubuntu-only include to main matrix (all platforms), replacing 3.9
  • Add 3.15 to include (ubuntu-only, pre-release)
  • Shift continue-on-error and build-deps install guard from 3.14 → 3.15 (3.14 is now stable)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Bump Python versions</issue_title>
<issue_description>Python 3.9 is EOL. Let's update the pyproject.toml and github actions to add Python 3.15 and drop support for Python 3.9 (similar to 750a189).</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Bump Python versions #192

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