TYP: simple return types from ruff
Ruff has many "unsafe" autofixes for simple return types.

I hope to enable `--unsafe-fixes` (for only the type annotations) after a few PRs.

Thanks @twoertwein  Is it needed to add `-> None` to test functions? (generally our `test_..` test functions don't have any type annotations at all, I think?) > Is it needed to add `-> None` to test functions? (generally our `test_..` test functions don't have any type annotations at all, I think?)

I think it would be good to exclude pandas/tests (and scripts/tests - happy to revert that) from these ruff fixes! I don't think it would add value for the tests (unless people prefer it for consistency).

However, manually adding annotations for tests can be valuable to selectively allow mypy to run on some tests to help with #45252. I believe most test files are completely ignored and unannotated functions are anyways skipped: so we could use `-> None` to run mypy on a subset of tests within a file.