Port flake8 to ArcanistExternalLinter

Summary:
Ref T3186. Brings another linter onboard. This one uses the stdin stuff.

The unit test was ostensibly broken so I fixed it, but that might just be some kind of version issue.

Test Plan: Unit tests.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3186

Differential Revision: https://secure.phabricator.com/D6802