Replace pytest-pep8 with pytest-flake8
Replace pytest-pep8 with pytest-flake8

pytest-pep8 isn't maintained any more and doesn't
work with latest pytest, so replace with the similar
pytest-flake8 package.

salilab/saliweb,salilab/saliweb,salilab/saliweb,salilab/saliweb,salilab/saliweb