mach: switch to uv for managing python venv (#34504)

This patch switches servo to use `uv` for both installing a pinned
Python version as well as installing the dependency packages using
`uv`'s pip compatible interface. It also introduces a new 'composite'
GitHub action to setup python in the different CI workflows.

There is no support for externally managed python installations and
virtual environments. These could be added in the future.

Fixes #34095

Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>

.github/workflows/mac.yml

@@ -77,18 +77,14 @@ jobs:
         if: github.event_name == 'pull_request_target'
         with:
           ref: ${{ github.event.pull_request.head.sha }}
-      # Python 3.13 breaks wptrunner, so pin the version until
-      # web-platform-tests/wpt#48585 is fixed.
-      - uses: actions/setup-python@v5
-        with:
-          python-version: '3.12'
+      - name: Setup Python
+        uses: ./.github/actions/setup-python
       - name: Run sccache-cache
         uses: mozilla-actions/sccache-action@v0.0.6
       - name: Install crown
         run: cargo install --path support/crown
       - name: Bootstrap
         run: |
-          python3 -m pip install --upgrade pip
           python3 ./mach bootstrap --skip-lints
           brew install gnu-tar
       - name: Build (${{ inputs.profile }})