Replace six in tests/unit/test_run.py

[cjmayo]

six not in requirements since:
b85542c ("Drop support of running under Python 2.7 (#2382)", 2022-07-25)

Thanks for contributing, make sure you address all the checklists (for details on how see development documentation)!

• ran the linter to address style issues (tox -e fix_lint)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
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