MAINT: use numpy.nanvar (requires numpy >= 1.8)

[jvkersch]

This came up in #10679.

Numpy 1.8 implements numpy.nanvar, which can replace pandas.core.nanops._nanvar when the minimum requirement for Numpy is bumped to 1.8 . The implementation of _nanvar in #10679 is in fact based on the corresponding implementation in Numpy.

I haven't checked whether other nanops can be replaced by a suitable Numpy equivalent.

[jreback]

@jvkersch I commented on the other issue. I think should simply do this now with a conditional on the version of numpy. just use not _np_version_under1p8.

alternatively

f = getattr(np,'nanvar',None)

and replace it with the current function

[jvkersch]

Will do, thanks!