refactor: add pre-commit hook pyupgrade, and run it

[consideRatio]

I think this hook is great. We have not run into troubles by using it in the JupyterHub org so far.

For example, all the changes by pyupgrade have always been logically equivalent it seems. If they hadn't been that, it would been a big problem as then it wasn't just formatting etc but actual changes to the code that mattered.

The changes in this PR are:

1. No longer passing "r" as an argument to open explicitly
2. Use of f-strings over %s formatted strings or .format() formatted strings.
3. class MockObject(object): became class MockObject:

Related

• Default to adopt pyugprade in our repos team-compass#453
• https://github.com/asottile/pyupgrade#readme

[consideRatio]

I re-reviewed the changes made by the pre-commit hook: pyupgrade which overall seems to be accepted by us in the JupyterHub team at this point and wen't for a merge. In a way, it is me reviewing the pyupgrade code as it if was a contributor.