ci: use uv

[wpbonelli]

Trialing uv to close #2416. Pixi is probably preferable, but for now, there is a blocker that I don't see a way around*. It's easy enough to switch once we are ready, and this gives us several CI benefits immediately:

• faster
• locked dependencies

I left the rtd.yml and examples.yml workflows untouched for two reasons:

• the model splitter needs pymetis, which is available for windows from conda-forge but not pypi
• the pypi vtk for windows crashed on one of the notebooks

These can be switched from micromamba to pixi later on, at which point we can consolidate on pixi for everything?

*Pixi has no way to tell uv to prerelease=allow, which is currently necessary because VTK has no valid 9.4+ official distribution for ARM macs on PyPI at the moment. I'd hoped PyPI dependencies could be overridden by conda dependencies, but pixi still tries (and fails) to solve them all for PyPI even if add them to pixi.tool.dependencies.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.3%. Comparing base (bb9824e) to head (d7a2e6b).
Report is 39 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2424     +/-   ##
=========================================
+ Coverage     68.4%   76.3%   +7.8%     
=========================================
  Files          294     294             
  Lines        59390   59853    +463     
=========================================
+ Hits         40652   45690   +5038     
+ Misses       18738   14163   -4575     

see 248 files with indirect coverage changes