Also lock down the version of pep8 in
tools/test-requires. python-swiftclient had a passing test suite
yesterday, but today a new and stricter version of pep8 came out, and
the test suite started to fail. Specifying a particular version of
pep8 will prevent that.
Change-Id: I9092d2225c01b99158229918c86b185cdac9d362