Add pyupgrade pre-commit hook

[jarrodmillman]

We've been using pyupgrade for several projects and I recommend it.

[choldgraf]

+1 from me - it should only run when Python versions are deprecated anyway, so I don't think it'll generate much noise right?

[12rambau]

I'm not in the Python game for long enough to realize how useful these tools can be. So nothing to add, if you think it's useful and if it's harmless let's make it happen

[jarrodmillman]

+1 from me - it should only run when Python versions are deprecated anyway, so I don't think it'll generate much noise right?

Correct. It shouldn't generate much noise and will ensure PRs use modern syntax and constructions. After we deprecate old Python versions, we can upgrade the flag and files in one commit, which we can add the commit hash to .git-blame-ignore-revs. For example, https://github.com/networkx/networkx/blob/main/.git-blame-ignore-revs

[jarrodmillman]

See #713.

[choldgraf]

Just realized we didn't merge this one in, sorry @jarrodmillman for dropping it, and thanks for the contrib!