Black, sort, testing

[ialarmedalien]

Extracting some of the linting from #6 to simplify that PR.

• ran flake8 and fixed obvious errors (e.g. unused imports, missing imports)
• ran black and sort over all files
• updated out-of-date actions in the main GHA workflow
• switched to using pytest to be able to generate XML coverage files that can be uploaded to codecov
• updated dependencies

[codecov-commenter]

Codecov Report

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