Surface NumPy FutureWarning about comparisons
Closes https://github.com/pandas-dev/pandas/issues/22698

Specifically, see
https://github.com/pandas-dev/pandas/issues/22698#issuecomment-458968300.

I don't think we need to worry about the others in https://github.com/pandas-dev/pandas/issues/22698#issuecomment-422446071 datetimearray and integer array. I couldn't reproduce the returning a scalar behavior with those.