mach: adopt uv and avoid system python (#34632)

This allows us to use `uv` for:
1. Installing a pinned Python version
2. Installing the dependency packages using `uv`'s pip compatible interface.
4. Bootstrapping `mach` without a Python installion on the host, using `uv
   run`

This change 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, #34547

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

.github/workflows/docs.yml

@@ -13,9 +13,10 @@ jobs:
     runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v4
+      - name: Setup Python
+        uses: ./.github/actions/setup-python
       - name: Bootstrap
         run: |
-          python3 -m pip install --upgrade pip
           sudo apt update
           python3 ./mach bootstrap --skip-lints
       - name: Set LIBCLANG_PATH # This is needed for bindgen in mozangle.