Update Crosscat to Python 3.

[emilyfertig]

check.sh passes, although there are still various breakages in examples and scripts (some of them involved missing files, which I wasn't sure how to fix).

Also, setup.py is broken (the call to git describe in get_version fails).

[usptact]

There are few simple fixes:

1. use python 3.8
2. create a tag $ git tag -a v0.1.56 -m "Emily Fertig py3"
3. in setup.py find the line desc = subprocess.check_output... and append .decode('utf-8')