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>
Servo already assumes the user has Python, this is the primary
way to make sure that Python 2 is preferred, and you should
get a sensible error message if you have python 3 but not 2.
But first, check that py.exe exists because if a system has only
Python 2.x only, it won't have it. If it doesn't, then try python.exe.
