Enable flake8 rules for unused things

Enables the following lints:

• F401 - module imported but unused
• F402 - import module from line N shadowed by loop variable
• F403 - 'from module import *' used; unable to detect undefined names
• F404 - future import(s) name after other statements
• F405 - name may be undefined, or defined from star imports: module
• F406 - 'from module import *' only allowed at module level
• F407 - an undefined __future__ feature name was imported