TYP: core.internals
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Annotating Block as `Union[np.ndarray, ExtensionArray]`, mypy gives tons of complaints about EA not having `.T`, so this adds that, then updates tests as appropriate