Remove redundant dependencies in test-requirements

Hacking 0.7 already pins down pep8, pyflakes and flake8 so they can be
removed from test-requirements as they are indirect dependencies.

Co-author: Joe Gordon <joe.gordon0@gmail.com>

Change-Id: Ia9f50e35f292ba35f8ca036cd66c930a3ad46ffd
Merge "Remove redundant dependencies in test-requirements"