losen pep8 version check
Summary:
pep8 has used both 2 (1.2) and 3 (1.2.1) digit versions. Losen
the version check to allow for both.
NOTE: This is the same regex as flake8.
Test Plan: arc unit with a 2 and 3 digit pep8 version on $PATH.
Reviewers: Blessed Reviewers, epriestley
Reviewed By: Blessed Reviewers, epriestley
Subscribers: joshuaspence, epriestley
Differential Revision: https://secure.phabricator.com/D11728