Fix numba deprecation notice

[mmcky]

The following https://intro.quantecon.org/inequality.html#measurement shows a numba deprecation warning from the quantecon package

[oyamad]

I believe it is coming from interpolation.
See #701.

[jstac]

@mmcky , I suggest replacing interp from interpolation with the function of the same name from NumPy.

[oyamad]

This is an issue of interpolation (and has been fixed by EconForge/interpolation.py#114).