ENH: Improve the scipy.spatial.qhull annotations

[BvB93]

Reference issue

Follow up on #13313.

What does this implement/fix?

This PR implements a number of improvements to qhull annotations (see commit messages).
Most notably among them are the addition of annotations for various attributes, all of which were previously missing.

Additional information

The behavior prior to this PR (due to the missing attribute annotation):

import numpy as np
from scipy.spatial import Voronoi

point = np.array([[0, 0], [0, 1], [0, 2], [1, 0], [1, 1], [1, 2], [2, 0], [2, 1], [2, 2]])
vor = Voronoi(points)

vor.regions  # error: "Voronoi" has no attribute "regions"

[BvB93]

scipy/stats/_qmc.py:892: error: unused "type: ignore" comment

There is currently an (unrelated) mypy error in stats._qmc.
I can address it here if so desired.

[rgommers]

LGTM, thanks Bas!

[rgommers]

This is useful - I can remember quite a few problems that could have been prevented by judicious use of @final.