Concatenation of series of differing types should lead to object
- [x] closes #21792
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
 
I see no simple solution on how to make a test for this without introducing a whole new ExtensionArray class that is actually of scalar type `'i'`.

cc @jreback I'm wondering a bit why this has not yet been a problem for https://github.com/pandas-dev/pandas/pull/21160 

I have check that this fixes the problems I'm seeing in `fletcher` on `int` and `float` dtypes.