MAINT: updating Cython imports

[tylerjereddy]

Related to this for newer versions of Cython/NumPy: scipy/scipy#14813

There are a few places we could probably add in np.import_array() just to be safe later on. Compare the output of git grep -E -i 'cimport numpy' and git grep -E -i 'import_array' to see the spots where we might add it in.

I don't think it is urgent though, and to be fair MDA already does the right thing in a few places based on those greps. Probably just a useful guard to add at some point.