usability of lint command

[chfw]

setup.py will cause flake8 to fail, leading to custom flake8 command, leading to missing flake8 installation -> install flake8 in a custom way, :/

[chfw]

https://travis-ci.org/moremoban/yehua/jobs/631623034 <- flake8 fails

[chfw]

with custom flake8 command, but flake8 needs installation. https://travis-ci.org/moremoban/yehua/jobs/631623882