Set Cython 'language_level` directive after 1.16 release.

[charris]

FutureWarning: Cython directive 'language_level' not set, using 2 for now (Py2).

The directive needs to be in the *.pyx files.

Reproducing code example:

import numpy as np
<< your code here >>

Error message:

Numpy/Python version information:

[sethtroisi]

I suspect this was handled by adding "-3" to cythonize.py in #13163? or by using the new version of Cython.

[rgommers]

indeed, thanks @sethtroisi