Add pyflakes tests

[virtuald]

pyflakes only detects things that are actually code errors (well... mostly), and it's pretty fast. Should add this in the future.

I think I would have it separate from the tests, as most of the things that pyflakes catches are things that are almost certainly errors.

[auscompgeek]

Note that pyflakes will emit warnings such as these too:

tests/pyfrc_test.py:6:1: 'from pyfrc.tests import *' used; unable to detect undefined names
tests/pyfrc_test.py:6:1: 'pyfrc.tests.*' imported but unused

flake8 assigns those error codes F403 and F401 respectively. ruff emits the first error, but not the second for some reason (which is probably a bug).