BUG: 1D slices over extension types turn into N-dimensional slices over ExtensionArrays
- [X] closes #42430
- [X] tests added / passed
- [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
- [X] whatsnew entry

This PR contains a 1-line patch for issue #42430, plus a regression test to ensure the issue does not recur.

Before this change, `ExtensionBlock. getitem_block_index()` would request 2D slices of a block's underlying extension array, even though the `__getitem__()` method of `ExtensionArray` is (currently) only supposed to support 1D slicing. After the change, `getitem_block_index()` requests 1D slices. The change also fixes some linter/type checking errors.