CLN, TYP Remove string type hints
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them

For some background: there was some consensus on doing this [here](https://github.com/pandas-dev/pandas/pull/36034#issuecomment-685186742)

Then [here](https://github.com/pandas-dev/pandas/pull/39270#pullrequestreview-571190631) there was the suggestion to use `ast.parse`, rather than regular expressions. I had a go at that, and made this into its own package: https://github.com/MarcoGorelli/no-string-hints , which I've included here as a pre-commit hook

The changes here are just a result of running `pre-commit run no-string-hints -a`, and then adding `from __future__ import annotations` to some files.