Upgrade asserts with teyit for Python 3.11 compatibility

[hugovk]

Use https://github.com/isidentical/teyit to upgrade unittest asserts for better error messages.

Also assertEquals has been removed in 3.11, so use assertEqual instead.

https://docs.python.org/3.11/whatsnew/3.11.html

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There are still some other things needed for 3.11 compatibility but this is a first step.

[hroncok]

What does "teyit" mean?

[hugovk]

It's this handy tool: https://github.com/isidentical/teyit

[stephenfin]

Can you rebase this? The branch doesn't have the Allow edits from maintainers flag set to let me do it.

[hugovk]

Hmm, strange the flag looks enabled for me:

Anyway, rebased!