Remove hardcoded version for pep8 from tools/test-requires

Remove hardcoded version for pep8 from tools/test-requires

In email discussions we decided that the master branch should always
pull in the lastest version of the pep8 tool, which keeps us closest
to the real pep-8 document.  Therefore, I'm removing the hardcoded
version here to match the lack of a pep8 version in tox.ini (which is
what the CI infrastructure uses when running the gating pep8 checks).

Note that your pep8 version will only update when you re-run
pip install --upgrade -r tools/test-requires .

Change-Id: I242cb042cff5f5969aa50d40312f87979191c7b7