Allow float window when using .rolling
- [ ] closes #12669
- [ ] tests added
- [ ] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

https://github.com/pydata/pandas/issues/12669

I just noted in the associated issue that that I think it would be better to raise than coerce integers.

It could be OK to accept floats if `int(window) == window`, but `window=2.5` should definitely raise an error.
 I agree we should just raise on non-int. I think was an oversight in the initial impl. yes its safe if `int(x) == x`, but more correct to raise `TypeError` as @shoyer suggests.
 competing PR is #12718 
 @gliptak want to update (also see my comments on the competeting PR).
