Add a run_constrained for libopenblas on openblas.

[isuruf]

To avoid libopenblas 0.3.6 and openblas 0.3.5 being installable in the same environment.

[ocefpaf]

@isuruf we have:

  - name: openblas
    ...
    requirements:
      run:
        - {{ pin_subpackage("libopenblas", exact=True) }}

Isn't that enough? Or should we do:

  - name: openblas
    ...
    requirements:
      run:
        - {{ pin_subpackage("libopenblas", exact=True) }}
      run_constrained:
        - libopenblas =={{ version }}

[isuruf]

Done in https://github.com/conda-forge/openblas-feedstock/pull/72