Add example for `noarch: python` in combination with version selectors

[pavelzw]

https://github.com/conda-forge/botocore-feedstock/blob/0ee2e3cdb522285b93a9897affe668990832f0bf/recipe/meta.yaml is an example where the noarch package depends on the python version. A similar use case would be the need to build two noarch: python packages based on the pydantic version you want to support. Would be cool to have this under advanced examples somewhere.