BUG: migrate away from broken optional dependency (pyregion -> regions)

[neutrinoceros]

PR Summary

Should fix #4234

[neutrinoceros]

two notes:

Turns out regions isn't a drop-in replacement for pyregion so we still need to adapt the code somehow

AttributeError: module 'regions' has no attribute 'parse'

Also, at least one of the notebooks that uses it in the docs is currently broken for a different reason:
https://yt-project.org/docs/dev/cookbook/fits_radio_cubes.html?highlight=fits%20radio

...
ValueError: Invalid vmin or vmax

This other problem is not my target here so if I can't find a solution I'll report it separately.
For information ping @jzuhone, who I assume is more knowledgeable than me on this part of the code

[neutrinoceros]

I may have opened this much too soon. Migration guidelines do not even exist yet, but here's where to follow progress on it, eventually: astropy/regions#488

[neutrinoceros]

With a little help from regions people (astropy/regions#490) I was able to complete the migration;
This can be checked with this snippet (slightly adapted from our docs):

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)

print(box_reg.quantities.extrema(("fits", "temperature")))

prj = yt.ProjectionPlot(
    ds,
    "z",
    ("fits", "temperature"),
    origin="native",
    data_source=box_reg,
    weight_field=("index", "ones"),
)  # "ones" weights each cell by 1
prj.set_zlim(("fits", "temperature"), 1.0e-2, 1.5)
prj.set_log(("fits", "temperature"), True)
prj.save("/tmp/")

which yields

(maybe the plot shouldn't have so much empty space in it but that's a different story, I think)

I will open this PR to review when I can verify that the docs build correctly

[neutrinoceros]

Not sure if this should be backported to the 4.1.x branch since regions isn't API stable yet, but I'll leave that decision up for discussion.

[matthewturk]

@neutrinoceros I think unless we get a bug report, we shouldn't at this point.

[neutrinoceros]

fine by me !

[neutrinoceros]

@matthewturk do you want to push the button ?