scikit-image no longer supports python < 3.6 / write our own contour-finding algorithm

[LoganAMorrison]

I'm getting build errors on python 3.5 when the system tries to install scikit-image. I get the following:

Running scikit-image-0.16.2/setup.py -q bdist_egg --dist-dir /tmp/easy_install-mn54hdhq/scikit-image-0.16.2/egg-dist-tmp-k530fmby
Python 3.5.6 detected.

scikit-image 0.16+ supports only Python 3.6 and above.

from TravisCI. I think we are only using scikit-image for contour-finding. Either we should remove support for python < 3.6 or write our own contour-finding algorithm. The latter isn't too difficult. The Marching Squares algorithm (https://en.wikipedia.org/wiki/Marching_squares )
is fairly straight-forward. We'd just want to make achieve similar performance as scikit-image.