Python tests should allow custom requirements

[hadim]

I think the python tests should allow custom deps spec. For example, in this feedstock I need to set the python version of a noarch package and it is not allowed:

tests:
  - python:
      imports:
        - pptx
      requirements:
        run:
          - python =${{ python_min }}

so I had to replace it with:

tests:
  - script:
      - python -c "import pptx"
    requirements:
      run:
        - python =${{ python_min }}

---

What do you think is the best way to address this?

1. allow requirements or requirements.run?
2. add python_version so only the python version can be customized?

Let me know, what you think is best here and I can submit a PR if you want.

[hadim]

xref conda-forge/python-pptx-feedstock#28