STYLE, TYP Turn black formatter off for overloads
Following on from the monthly meeting's discussion, some things were suggested:

- turning off the black formatter for the overloads, keeping them in `.py` files (what I've done here)
- moving overloads to `.pyi` files, writing a script to keep them in sync (I tried that this, and it seems that `mypy` behaves a bit differently with a `.pyi` file than it does with a `.py` file - will look into this more closely, but it'll take more time)
- keeping overloads out of `.py` files and "injecting" them before running `mypy` via a script - IMO this might be slightly disruptive to the usual workflow of just running `mypy pandas`, but other than that seems like a potentially good idea
- doing away with overloads, and just using `cast` and `assert` - TBH this strikes me as more error-prone, and if pandas will expose types to users, then I think it'd be better to have the correct type revealed rather than having them do `cast` / `assert`

Even if `inplace` is removed in version 2, there are other places in the codebase where there's a parameter which changes the return type, so I do think finding a solution here is worthwhile.

For now, I've gone with the first solution, though I will give some more thought to the second one