Add pytest-cov test.txt dependency list

[bobleesj]

We shall use pytest-cov instead of using coverage to run pytest. The previous codecov 0% error occurred because the tests folder has been moved to the top directory level and coverage isn't able to import diffpy. due to the namespace.

So the GH actions can be modified

from:

        run: |
          coverage run -m pytest -vv -s
          coverage report -m
          codecov

to:

        run: |
          pytest --cov
          coverage report -m
          codecov

Reference:

• Create codecov.yml for coverage testing (Do not merge) diffpy.snmf#86

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[sbillinge]

Thanks for the detailed explanation