Remove py2-flake8_extended job

Removing flake8-builtins from the required packages in flake8_extended job leaves only flake8-isort as the extended checks. At this point, I'm not sure it makes sense to keep it like that and went ahead and propose removing it altogether.

Alternatives are either to maintain py2-flake8_extended with only isort check added, or to remove py2-flake8_extended but moving the isort check to the non-extended flake8 jobs.

Closes #59 (closed)