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>