Add requirement.constraint key to support package-wise constraints

[ryanking13]

Resolve: #87

This PR adds a new key in meta.yaml file: requirement.constraint.

In the build time, pyodide-build will generate a new constraints.txt file in the build directory, and update the PIP_CONSTRAINT env variable to point that file. The new constarints.txt file contains host_constraints_file + constraints defined in the recipe.

• changelog

[hoodmane]

Thanks it looks very useful. It might be nice to add automation to override rust constraints later too.

[hoodmane]

Maybe let's just call it _get_constraints()?

[ryanking13]

Thanks for the suggestion. I renamed it to _create_constraints_file as I thought that using the word get is confusing regarding that it creates a file internally.

[hoodmane]

Maybe numpy and scipy are bad examples though since realistically this is broken I'd assume?

[agriyakhetarpal]

I mostly agree with the functionality here, but I would like to suggest that we have simpler syntax – instead of having a constraint: field containing keys that are the same as dependencies and are therefore redundant, could we add constraints to the dependencies: key itself? i.e., we can have two ways:

requirements:
  constraint:
    - numpy <2
  executable:
    - rustup
  host:
    - numpy
  run:
    - numpy
    - scipy

and

requirements:
  executable:
    - rustup
  host:
    - numpy <2
  run:
    - numpy >= 1.24
    - scipy >= 1.12.1, <1.14

The second option seems better, as it's closer to the conda-forge format. Also, this would allow adding constraints separately to the host and runtime dependencies (useful for build-time dependencies such as NumPy in this example), while constraint: seems to be applicable for both host and runtime dependencies simultaneously.

[ryanking13]

@agriyakhetarpal Thanks for the suggestion! I generally agree that it would be better to use the build.host key and build.run key to specify the constraints. It also aligns with the conda's meta.yaml specification.

But the problem is that currently, the requirement key in Pyodide's meta.yaml file does not align with the build dependency used by pypa/build. So it is a bit tricky to integrate it with pypa/build. So let's go as-is and think of a better way later.