STYLE remove --keep-runtime-typing from pyupgrade
xref @simonjayhawkins 's comment: https://github.com/pandas-dev/pandas/pull/40545#discussion_r606360410

Task is:
- remove `keep-runtime-typing` from:

https://github.com/pandas-dev/pandas/blob/cc84a2383c641998d4817cfaeb33c64f017b7454/.pre-commit-config.yaml#L57

- run `pre-commit run pyupgrade --all-files`
- run `pre-commit run flake8 --all-files`, fixup any errors (probably some about extra imports)

Finally, run `pre-commit run --all-files` (this may take a few minutes), fixup any hooks which don't pass, and when everything passes, open a pull request :rocket: 