Add pyright as well as tests and infrastructure inspired by bugbear

[jakkdl]

I didn't find a way to configure pyright in setup.py, opted to use a pyproject.toml instead of a pyrightconfig.json.

With pytest-xdist commented out passing -n auto errors out. Commented out the parameter, but probs cleaner to uncomment both lines or remove both.

Could opt to explicitly list files to avoid checking setup.py instead of adding the check for __version__ in it.

[jakkdl]

All failed checks are for stuff I didn't touch in this pull request, opening another one that works on flake8_trio.py and adds basic tests.

[Zac-HD]

• pyright config lgtm, discovery is less error-prone than explicit lists
• let's leave xdist commented out for now; we'll uncomment once we have ~10 tests and concurrency is worth the overhead
• obvious test failures are obvious, feel free to fix them here or in another PR as you prefer.

[jakkdl]

kk, let's just do that here then. I haven't delved into understanding the testing setup that bugbear uses, so just doing basic tests for now.

[jakkdl]

Okay, cribbed bugbear's testing setup. Had to replace the namedtuple+partial solution to get it to pass pyright, though likely possible to get a cleaner solution. Dropped stdin support for the filename parameter that bugbear had, was way too much of a hassle to get that covered and I'm not sure how big the need for it is. I've never done fuzz tests before, but copy-pasted to my heart's content and they seem to do something. Added back xdist since fuzzers started taking time.

[jakkdl]

fixed all comments, code starting to look pretty :sparkle: