Start managing Python version within the action

[jku]

Currently we require the user to manage the Python installation: Often the runner provided Python just works but Windows runners currently default to 3.9 which is incompatible.

This leads to issued like #236, #235. PR #238 improves the situation slightly by making checks more strict.

We may actually want to start to manage the python version in the action itself. Options include:

• Use composite actions and invoke actions/setup-python ourselves. This has some side effects: the user might not expect the default python changing when they run gh-actions-sigstore-python...
• use uv to choose the python we use. This seems like the better option

[jku]

#242 implements the uv-option -- it's not as trivial as it should be but I still like it