Hello,
Just putting my two cents here.
I believe we have something like this done. Take a look at
Coala and
Gitmate. The Gitmate is a bot which can check your PRs for all the stuff( PEP8, Lint Errors and such ) and tell the contributor about the exact problems in the code.
The Gitmate requires coala to run, maybe we can discuss with Coala devs regarding the integration of the bot, since the bot can do more than just PEP8 checks.