Switch to uv for all of our Python scripts (#7242)

This removes the need to install any specific version of Python or
figure out how to configure it by instead asking users to install `uv`
and letting it manage Python. Among other advantages, `uv` is designed
to be fast enough to embed directly into our scripts.

We were already using this in `bench_runner.py` so that the script could
import non standard library dependencies. Moving to it for the rest of
our Python unifies the approach and will also enable dependencies
whenever needed.

I've left `github_tools` alone as it has special handling with its own
Bazel setup.

I've updated the contributing tools to explain the approach here.

.github/workflows/pre_commit.yaml

@@ -40,6 +40,7 @@ jobs:
             oauth2.googleapis.com:443
             objects.githubusercontent.com:443
             pypi.org:443
+            raw.githubusercontent.com:443
             registry.npmjs.org:443
             release-assets.githubusercontent.com:443
             releases.bazel.build:443
@@ -48,7 +49,6 @@ jobs:
             www.googleapis.com:443
 
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
-      - uses: actions/setup-python@a309ff8b426b58ec0e2a45f0f869d46889d02405 # v6.2.0
 
       # Ensure LLVM is set up consistently.
       - uses: ./.github/actions/build-setup-common