Use pyupgrade

[kurtmckee]

This PR introduces the following changes:

• Immunized usage of io.open() so it doesn't get rewritten by pyupgrade
• Ran upadup to update pre-commit hooks' additional_dependencies
• Ran pre-commit run -a to fix the docs based on a much newer black version used by blacken-docs
• Added pyupgrade as a pre-commit hook, with Python 3.7+ idioms enforced

Closes #1020

Describe the changes

The related issue or a description of the bug or feature that this PR addresses.

Tasks

• Unit tests added that reproduce the issue or prove feature is working
• Fix or feature added
• Entry to release notes added
• Pre-commit CI shows no errors
• Unit tests passing
• For documentation changes: The Read the Docs preview builds and looks as expected

[mrbean-bremen]

This also merits an entry in the release notes (under the "infrastructure" heading).
Thanks for that, I didn't expect that you follow up so fast after my comment!

[mrbean-bremen]

Thank you!