Revert "add mirror requirements list"

This reverts commit b1101620fc0ba1938708b2e93f529341a016f0ae.

The new thinking is that hacking itself should enforce that the
three hacking requirements are not in project's test requirements.

Change-Id: I7167968c272552838985fe8b9241549abae0da18
Bump pep8, pyflakes and flake8 versions

Since newer versions of each package can either break
hacking(flake8, pep8) or introduce new rules and because we gate on
the output of hacking, we pin these dependencies to specific versions.

* Set pyflakes to the newest version: 0.8.1
* Set flake8 to to the newest version: 2.1.0
* Set pep8 to 1.4.6, since newer version (1.5.1) breaks hacking and will
  require additional work to support it. Will be addressed in a future
  patch.

Add the new versions along side the old versions so we don't create a
backwards incompatible global requirements change.

To support hacking change: Ia3495102a9d096c09802b9c079a8dc7ec1f65530

Change-Id: I5bebee5af87a75781b840daead61436af0d6ab3a