cborutil: port to Python 3
The only problem lurking in here was sorts of mismatched types. The
sorts are only for output stability in our tests (sigh), so we just
build a phony sort key using the __name__ of types so that we only
compare like types against each other. By pure luck, my awful sort key
matches the behavior we get "for free" in Python 2, so no test output
changes.
Differential Revision: https://phab.mercurial-scm.org/D3504
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