BUG: broken optional dependency (pyregion)

[neutrinoceros]

Bug report

Bug summary

pyregion is being used sporadically in a couple docs notebooks and is a docs-only dependency to yt. It's also quite fragile as no wheels are available and pip fails to build the current latest version (2.1.1) from source.
The project is targeted for retirement astropy/pyregion#153, so we should move to regions when possible (maybe it's already the case).

[neutrinoceros]

Here's a minimal example of how the ds9_region function is supposed to be used

import yt
from yt.frontends.fits.misc import ds9_region

ds = yt.load("grs-50-cube.fits.gz")
region = 'galactic;box(+49:26:35.150,-0:30:04.410,1926.1927",1483.3701",0.0)'
box_reg = ds9_region(ds, region)