Make flake8 an optional dependency

[jakkdl]

This has been lying around mostly-finished in a branch for a while, so I thought I'd clean it up and push it.

Writing our own parser of noqa was easy, but we still rely on flake8 for parsing config files. This means a subset of users that:
a. use flake8 config
b. do not install any other flake8 plugins or flake8 itself in that venv, and
c. run the linter with flake8-async

will ~silently get a large change in behavior and they have to go look up the changelog to see they now need to install flake8-async[flake8]. If they don't do c they'll get a more obvious error that flake8 is not available.
(only doing b and not c will also give a fairly obvious error)

I'm not sure how many people that will affect, but it might be worth quickly searching pyproject.toml or .flake8 for the string ASYNC when flake8-async is invoked and give a big warning. Maybe even make it a two-step deprecation+release where we add the extra+warning now, and remove flake8 as a dependency in a couple weeks.