tox: Add flake83 environment to run flake8 under Python 3

The flake8 env is now explicitly for Python 2. This will allow
us to run flake8 on both 2 and 3 together.

Change-Id: I3502b8266d735577089a3782d7e3f2c08c8c526e
Merge "tox: Add flake83 environment to run flake8 under Python 3"