CLN,TYP Remove string return annotations

- [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

this was just done with
```python
    parser = argparse.ArgumentParser()
    parser.add_argument('paths', nargs='*', type=Path)
    args = parser.parse_args()
    for path in args.paths:
        text = path.read_text()
        text = re.sub(r'-> "(\w+)"', '-> \\1', text)
        path.write_text(text)
```

There's other to do (which I'll get to in other PRs), but this already gets rid of a big load of them
Not a huge deal, but I like the string versions since they make the dependency structure clearer, i.e. I’m not going to incorrectly infer that Series is in the namespace. Sure, but you can't click and go to a definition if it's a string version (this is my main reason for wanting to make this kind of change)

I think it only makes the dependency structure clearer if string ones are used consistently (currently there's a mix) - up to you anyway, my preference here isn't too strong > Sure, but you can't click and go to a definition if it's a string version (this is my main reason for wanting to make this kind of change)

some discussion in https://github.com/pandas-dev/pandas/pull/36034#issuecomment-684972993


> I think it only makes the dependency structure clearer if string ones are used consistently (currently there's a mix)

agreed. You can (and people have) quote types even if the type is imported at runtine, or quote the whole type expression such as `"List[grouper.Grouping]"` which the PR resolves.

"we" agreed (https://github.com/pandas-dev/pandas/pull/36034#issuecomment-685186742) on going this route. Do we want another discussion? > Do we want another discussion?

Nope, I'm pretty happy with my "defer to Simon when it comes to annotations" policy. OK, merging following https://github.com/pandas-dev/pandas/pull/39174#pullrequestreview-568442160 and https://github.com/pandas-dev/pandas/pull/39174#issuecomment-760992916 Thanks @MarcoGorelli 