FIX: Stable degeneracy

[nkeim]

This resolves #776 in most if not all cases, by making the order of a set of Point objects non-random.

While the order of a set is never guaranteed, it is based on the objects' hash values. Starting with Python 3.3 these hashes were randomized as a security measure. This issue was causing repeated linking runs to resolve degenerate cases randomly, even within the same session. This PR makes Python use the Point's uuid attribute (basically, its serial number) instead.

This PR adds tests to make sure that degenerate linking cases (with and without memory) always resolve the same way within the same session. It does not ensure that results are consistent across linkers or across sessions. (It turns out this requires a change to the linking algorithm itself, so it will be a separate PR.)

[nkeim]

I should note that besides fixing the issues with GitHub actions, it's important to evaluate the effect of this PR on performance. It's easy to see how using a custom hash could dramatically slow down linking. I would love for someone to check this with their own data!