Add complex number support to isinf

[kgryte]

This PR

• adds complex number support to isinf by documenting special cases. Namely, as long as either a real or imaginary component is positive or negative infinity, the result should be True, including if the other component is NaN. This follows C (one infinity model), Python, Julia, and NumPy.
• updates the input and output array data types to be any numeric data type, not just real-valued data types.

Reference

>>> z
(inf+nanj)
>>> cmath.isinf(z)
True

>>> import numpy as np
>>> z
(inf+nanj)
>>> np.isinf(np.array([z]))
array([ True])

julia> z
Inf + NaN*im

julia> isinf(z)
true

[rgommers]

LGTM. This seems consistent with existing practice, and I don't think there were any remaining concerns with that behavior. In it goes, thanks @kgryte