Move pep8 configuration to setup.cfg

pep8 documentation says that .pep8 files are deprecated, but it will
automatically read settings from a [pep8] section in setup.cfg. I've
moved the .pep8.ini content to setup.cfg, which means that the pep8
command won't need the --config parameter anymore.

We run Flywheel tests on our Jenkins server at Runscope, and we made this
change to make Flywheel's packaging a little bit more compatible with our
Jenkins setup.