Use uv to install python dependencies in tests

[thomasjpfan]

Describe your changes

I noticed that caching is not working for one of the MacOS runners. This PR changes the python setup code to use uv instead to see if it's better.

Checklists

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Compatibility checklist

Check these boxes or delete any item (or this section) if not relevant for this PR.

• Client+Server: this change is compatible with old servers
• Client forward compatibility: this change ensures client can accept data intended for later versions of itself

Note on protobuf: protobuf message changes in one place may have impact to
multiple entities (client, server, worker, database). See points above.

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Release checklist

If you intend for this commit to trigger a full release to PyPI, please ensure that the following steps have been taken:

• Version file (modal_version/__init__.py) has been updated with the next logical version
• Changelog has been cleaned up and given an appropriate subhead

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Note

Switches the composite GitHub Action to install and cache Python dependencies with uv instead of pip/actions/cache.

• CI / GitHub Actions:
  • Replace actions/cache with astral-sh/setup-uv to enable dependency caching via uv (enable-cache: true, cache-dependency-glob for requirements.dev.txt and pyproject.toml).
  • Install dependencies with uv pip install -r requirements.dev.txt and set UV_SYSTEM_PYTHON=1 (remove manual pip upgrade/install).
  • Keep Python setup via actions/setup-python unchanged; pin uv version to 0.9.7.

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