Format code with black

[IvanIsCoding]

Format Python code with black to align code formatting with recent changes to other Qiskit repositories.

This formatting changes a massive amount of test files, so I will highlight the files that actually need to be reviewed:

• tox.ini for adding tox -eblack and making flake8 agree with black
• pyproject.toml for adding black settings
• CONTRIBUTING.md with instructions to run black for new contributors
• .github/workflows/main.yml for checking black codestyle in CI

[coveralls]

Pull Request Test Coverage Report for Build 820926823

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 96.578%

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Totals
Change from base Build 810011546:	0.02%
Covered Lines:	6942
Relevant Lines:	7188

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💛 - Coveralls

[mtreinish]

Overall this LGTM. Personally I've resisted using black here because I personally really don't like some of it's style choices it makes. But, I think we've already (for quite some time) reached the point where retworkx is no longer solely my domain. With that in mind I think using black to handle the code formatting makes sense since it simplifies the code formatting for contribution in the same way rustfmt does (also the style is a closer match to rustfmt).

I just left a couple of comments inline. Of those comments most are just idle thoughts or questions for the future (mostly around line length) the only required changes are in the tox.ini.
Aside from the inline comments though, you will also need to manually update the lint github actions job. We don't use tox for that job because I wanted to run cargo directly for the well formatted compiler error message and we already build twice, so doing it again for tox seemed unnecessary. I guess you could update the job either to just explicitly run black --check --diff or change the lint jobs to be tox --skip-pkg-install -elint either would work (the later might be better to be more consistent between local and ci, especially with a pinned black version).

[mtreinish]

Wow this is pretty ugly.

[IvanIsCoding]

Perhaps the beauty is that we didn't have to format it ourselves!

[IvanIsCoding]

I have addressed some comments and:

• black version is now fixed
• tox -elint and tox -eblack share the same environment
• black is executed in CI. I opted to add a new command because I found it simpler
• Line lenght is now 80

With regards to line lenght: I agree with the idea of using the same line lenght for cargo fmt and black, consistency is a great idea!

It is easier though to set black to use line lenght of 80 than to set cargo fmt to use line length of 100. We have way more Rust code than Python code, so I tried to be mindful and minimize the diff. Also, most of our Rust code is core functions while most of our Python code is test cases. So let's keep Rust as it is 😃