ENH: Add annotations for scipy.spatial.cKDTree

[BvB93]

Reference issue

n.a.

What does this implement/fix?

This PR adds annotations to the cKDTree and cKDTreeNode classes for the purpose of static type checking.

Additional information

• There were two methods where different output types are returned for 1D vs >= 2D array-likes (see below).
  As we cannot describe array shapes/dimensionality as of yet, the output type is set to Any here.
• There are 2 checks for 0D array-likes, the latter currently excluding 0D arrays as (again) we cannot type
  array shapes/dimensionality yet. This does mean that there is some small room for a false positive here,
  though these are very much edge cases in my opinion.

[BvB93]

Tests failures seem to be due to #14014 thus far.

[tupui]

IMO we should follow what we did in #13833. Have inline typing and also for int,float use the things from scipy._lib._util that this PR will introduce.

[BvB93]

As far as I'm aware inline type hinting is not possible here, as the source is based on cython .pyx files.
Should be doable for KDTree though, but that's beyond the scope of this PR.

[tupui]

As far as I'm aware inline type hinting is not possible here, as the source is based on cython .pyx files.
Should be doable for KDTree though, but that's beyond the scope of this PR.

Ah right, I did see this was for cython files.

[tupui]

As discussed during the meetup. This is ready so I am merging. Thanks @BvB93!