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: Ia4dc72581e4c0bce7c452969078c7abf18abdd4c

tox.ini

@@ -163,3 +163,22 @@ deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt
   -r{toxinidir}/requirements.txt
+
+[testenv:dev]
+# run locally (not in the gate) using editable mode
+# https://pip.pypa.io/en/stable/reference/pip_install/#editable-installs
+commands =
+    pip install -q -e "git+https://git.openstack.org/openstack/neutron#egg=neutron"
+
+[testenv:py3-dev]
+basepython = python3
+commands =
+    {[testenv:dev]commands}
+    {[testenv]commands}
+
+[testenv:pep8-dev]
+deps =
+    {[testenv]deps}
+commands =
+    {[testenv:dev]commands}
+    {[testenv:pep8]commands}