[canismarko]

While working on #1188, I wound up adding support for running the tests with Pixi. I figured I'd submit those changes too, since it would make it easier for me (and maybe others) to run the tests in the future.

Pixi has two ways you add support: pyproject.toml, or a separate pixi.toml file. I went with the pyproject.toml route since it lets pixi re-use the dependency groups inside pyproject.toml.

This PR shouldn't affect the ability to install apstools with mamba, conda, pip, etc.

This PR does not change anything about how github workflows are run. I'd be willing to switch the github code.yml to use pixi in the future if we find that there's enough benefit in having a lockfile.

This PR does not use Pixi for managing any external dependencies for the tests, like the GP and ADSIM IOC's.

[canismarko]

The killer feature here is the lockfile. pixi.lock specifies exactly which version of each package is installed in each environment, and so anyone running the tests with Pixi will get exactly the same versions. If packages are upgrade, the environments are re-solved, the lock file is updated, and the new pixi.lock gets committed alongside the update pyproject.toml.

[prjemian]

LGTM