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
Merge "Move pep8 to just 1.5.7"