add local tox targets for pep8 and py3
add local tox targets for pep8 and py3

This patch adds local tox targets for pep8 and python 3 as per [1]
that will install dependencies in editable mode.
To run them use the pep8-dev and py3-dev targets respectively.

[1] https://etherpad.openstack.org/p/neutron-sibling-setup

Change-Id: I27fd4d41fa66924507d58a980813d150b5299e1b

openstack/networking-bgpvpn,openstack/networking-bgpvpn,openstack/networking-bgpvpn