Additions to allow runtests.py to run pyflakes

Before adding pep8, mccabe and pyflakes I was seeing errors like

`ImportError: No module named pep8`

`pkg_resources.DistributionNotFound: mccabe>=0.2.1`

`pkg_resources.DistributionNotFound: pyflakes>=0.8.1`

Pull request contains these as additions to requirements.txt in case they are needed.