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]

I assume in the future this would replace testing.yml?

[glemaitre]

Yep it was to have a first push to not remove the previous file :)

[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.

[glemaitre]

We might need to setup the secret in GitHub if it does not exist.

[jeromedockes]

does this mean we are now running the tests from the source directory? I think that's not ideal with the current layout of the package because it could end up being imported from the pwd? or maybe I am missing something

[jeromedockes]

I guess there's no way to specify this environment with pixi?

[glemaitre]

Here, it does not make sense to use pixi because you don't want to lock the nightly builds.

In this manner, we also test that our pip command of pyproject.toml work

[jeromedockes]

thanks @glemaitre ! this simplifies the CI configuration while fixing several issues