nix: add black and isort to the closure

[danieldk]

What does this PR do?

To make sure that everything is formatted with the same black version as CI.

I sometimes use isort for new files to get nicely ordered imports, so add it as well. Also set the isort configuration to format in a way that is compatible with black.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
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[drbh]

looks good/reasonable to me 👍 (still learning nix; but changes seem isolated and correct)