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.

proposals/scripts/new_proposal.py

@@ -1,4 +1,8 @@
-#!/usr/bin/env python3
+#!/usr/bin/env -S uv run --script
+
+# /// script
+# requires-python = ">=3.11"
+# ///
 
 """Prepares a new proposal file and PR."""