Get test Python version from the HA Core repo.

[ademuri]

This changes the update-test-matrix.py script to fetch the required Python version for a given HA Core version by fetching the pyproject.toml for that version. It updates the test matrix with the updated script. It also updates the Python version used for building the Docker image and devcontainer to 3.14, since that's the version now required by HA Core.

This fixes a CI breakage I saw on a draft pull request:

+ uv pip install -e core/
  × No solution found when resolving dependencies:
  ╰─▶ Because the current Python version (3.13.12) does not satisfy
      Python>=3.14.2 and homeassistant==2026.3.0.dev0 depends on
      Python>=3.14.2, we can conclude that homeassistant==2026.3.0.dev0 cannot
      be used.
      And because only homeassistant==2026.3.0.dev0 is available and you
      require homeassistant, we can conclude that your requirements are
      unsatisfiable.