Add `requirement.constraints` key in meta.yaml?

[ryanking13]

For now, we are using a global constraints.txt file for building packages, but as the number of recipes grow, sometimes applying the same constraints to all packages doesn't fit.

For instance, pyodide/pyodide-recipes#64 (comment) had issue with the pined maturin version and the version that the package requires.

So, I think it would be useful to have a key in the meta.yaml that specifies the build constraints for the package:

requirements:
  constraint:
    - maturin < 1.8  

and apply it only for that package.

cc: @chrysn