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/actions/setup-python/action.yml

@@ -0,0 +1,20 @@
+name: Setup Python and uv
+inputs:
+  skip-python-setup:
+    required: false
+    description: "Whether to skip installing python using Github's `setup-python` action"
+    default: false
+runs:
+  using: "composite"
+  steps:
+    # Use the setup-python action to take advantage of the cache. uv will
+    # symlink to this version.
+    - name: Setup system python
+      if: ${{ inputs.skip-python-setup != 'true' }}
+      uses: actions/setup-python@v5
+      with:
+        python-version-file: '.python-version'
+    - name: Install uv
+      uses: astral-sh/setup-uv@v4
+      with:
+        version: "0.5.6"