Fix np.bool dtype deprecation

[jproctor10]

plotly.py/packages/python/plotly/plotly/express/_imshow.py

Line 349 in 03979d1

if img.dtype == np.bool:

This line is causing warnings related to https://numpy.org/devdocs/release/1.20.0-notes.html#using-the-aliases-of-builtin-types-like-np-int-is-deprecated

[nicolaskruchten]

Closed by #3705.