testing-framework

We need a page in the docs that tell users that to mock they should use from unittest import mock.

We should note that the mock.patch decorators don't currently work, and that we need to use the context managers instead.

Right now (in precommit.py) we only enable a few checks (E9, F63, F7, F82).

Looks like there are a lot of other useful ones in there; investigate moving to a set of blocklisted checks.

Right now we just use the standard C random library, but really it'd be nice if the brute force fuzzer could do consistent tests across platforms, for various purposes. We probably need our own PRNG, something like:

https://www-cs-faculty.stanford.edu/~knuth/news02.html#rng

or

https://github.com/cmcqueen/simplerandom

I'm not sure what's best. We'd like decentish quality, maybe, and d