Sync Python pin and lower bound

[jl-wynen]

These are two connected sub issues:

.python-version files

Can we create the project-level .python-version files from

[dependencies]
python = ...

in the top-level pixi.toml?

OR do we even still need those files?

From #62 (comment):

Can we sync .python-version files with pixi.toml?

Probably needs to be a script/pre-commit bit.

requires-python in pyproject.toml

Can we / do we need to sync the Python bounds in the projects' pyproject.toml files? I'm thinking we might not need to do that as long as pixi.toml pins a version for development and CI and as long as we have lower bounds tests that use the version bound in pyproject.tonl.