Decouple end user dependencies from development dependencies in pixi.toml

[kklein]

Status quo

As of now, the dependencies section in pixi.toml looks as follows:

https://github.com/Quantco/metalearners/blob/main/pixi.toml#L13-L45

Notably, it includes dependencies, such as pytest, which are required for development of the metalearners library but not for its use by an end consumer.

This is plainly apparent when looking at the run requirements of metalearners' conda-forge recipe:

https://github.com/conda-forge/metalearners-feedstock/blob/main/recipe/meta.yaml#L24-L31

Objective

Separate development dependencies from end user dependencies in pixi.toml. This will make it

• easier to keep the conda-forge recipe in a correct and consistent state and
• more transparent if a code change imposes the burden of an additional dependency on the end user.