gh-35385: src/tox.ini: Add cython-lint
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### :books: Description
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Adding the command `sage --tox -e cython-lint`.
See https://pypi.org/project/cython-lint/
There are many failures, so it is not run by default.
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### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/35385
Reported by: Matthias Köppe
Reviewer(s): Frédéric Chapoton
