Setup check to install/enable pygments

Summary: Fixes T6533 - actively prompt the user to install and enable pygments

Test Plan: uninstalled / disabled pygments and checked setup checks

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6533

Differential Revision: https://secure.phabricator.com/D11048