`np.bool8` is now deprecated

[kylebutts]

I'm using the most recent version of numpy and it's causing errors with this single line of code:

pytwoway/pytwoway/preconditioners/ichol.py

Line 38 in 7208bda

n, np.bool8

Switching just to bool instead of np.bool8 should fix the problem. See https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations