Allow to lint Python code

Summary:
For simplest scenarii, flake8 allows to lint Python file.

For more complex ones, all to handle virtualenv is there too.

Fixes T1810.

Test Plan:

• arc lint --everything
• create and activate a virtualenv

Reviewers: dereckson

Reviewed By: dereckson

Subscribers: Nasqueron Docker deployment squad

Tags: Docker images

Maniphest Tasks: T1810

Differential Revision: https://devcentral.nasqueron.org/D2948