MAINT use pixi to manage the CI dependencies

[glemaitre]

closes #708 , closes #899, closes #860

Let's see if we can use pixi to have kind of lock files system to not have failing CI when contributing.

Here, I wanted to check that everything works before to go further.

It should be noted that the pixi.toml file can be merged with pyproject.toml and I don't think that we need anymore the setup.cfg as well.

[jeromedockes]

while you're tinkering with the CI you may want to see #899 as well

[glemaitre]

I think the CI is failing because the "Required" CI are failing because I removed the script.