Earth mover's distance and POT dependency

[sdmccabe]

Ideally we would deprecate the POT dependency and use ortools instead, since it's a headache for the requirements.txt

[leotrs]

To be totally fair, the problem comes from a bug in pypi, not pot, as documented here.

I agree the way to go is to replace POT.

[sdmccabe]

Related to the POT dependency is that we still have == in our requirements for Cython.

[leotrs]

There's been an update on their side, mostly on the documentation side of things. They closed their open issue, though it is unclear that their new PR actually solves our problem.