New pixel graph implementation

[jni]

This is a major change in skan that removes old junction node handling in favour of the new MST junction "cleanup". Building the graph is much faster and we preserve pixel-perfect correspondence between the skeleton graph and the original image.

• Use new pure Python method to generate pixel graph
• Remove JunctionModes and unique_junctions args
• Remove unnecessary junction mode tests; fix tinycycle test
• Use new tuple coordinates in draw tests
• Update pipe to use Skeleton class
• Update skeleton class testing

[GenevieveBuckley]

Tests are failing because skan isn't able to import a private function from scikit-image. I suspect that the CI is using a newer version of scikit-image where the location of this private utility function has moved.

  File "/opt/hostedtoolcache/Python/3.9.9/x64/lib/python3.9/site-packages/skan/csr.py", line 6, in <module>
    from skimage.morphology._util import _raveled_offsets_and_distances
ImportError: cannot import name '_raveled_offsets_and_distances' from 'skimage.morphology._util' (/opt/hostedtoolcache/Python/3.9.9/x64/lib/python3.9/site-packages/skimage/morphology/_util.py)

[GenevieveBuckley]

Hm, maybe that's not actually the reason. The _raveled_offsets_and_distances function is in the expected place in the scikit-image main branch.

[GenevieveBuckley]

Turns out it's the other way around. The function _raveled_offsets_and_distances was introduced very recently in scikit-image/scikit-image#5602, so currently it is only available from scikit-image version 0.19.0rc0.

So this PR should also update the requirements to specify scikit-image>=0.19.0 (and we'll need to wait a little bit for that release to become available). Sorry Juan, I think you knew all of this already!

[jni]

Thanks for reminding me @GenevieveBuckley! tbh I was writing this a while back and I would have had to go figure it out 😂 Probably I'll just vendor the function.