Pyupgrade as a code modification backend

[akaihola]

Similar to flynt and isort, pyupgrade could be supported as a backend tool for making modifications to Python code.

pyupgrade describes itself as:

A tool (and pre-commit hook) to automatically upgrade syntax for newer versions of the language.

A previous effort to make Darker support pyupgrade was started already in August 2020 by @Carreau in #51. Two years later, support for flynt was being developed (PR #308), and the thought was that the exact same mechanics of running the backend could be applied to pyupgrade.

All the relevant discussion is still available in the closed issue #51, and the original branch has been copied from @Carreau's fork into the main Darker repository as tree/pyupgrade