Updated openstack/openstack
Project: openstack/requirements  3d28e020d05a032d71810316844278a3929312db

Move pep8 to just 1.5.7

pep8 is just a bug fix release beyond, there are no new rules that were
not in pep8 1.5.6.

This move to 1.5.7 will help us move to flake8 2.2.0 which has no
additional rules added since flake8 2.1.0 but has parallelization turned
on by default.

Change-Id: I872a180eb066b4b85620bc03bce319345d004129