add py3-dev tox target and update lower constraints zuul job

This patch adds a py3-dev tox target for running py3 UTs locally.
It also updates the local .zuul.yaml for a neutron-vpnaas specific
lower constraints job that ensure neutron/master is installed.

Change-Id: Iab9f134b17b56d765d5ac523d9b5a4b4d4e00fb7
Partial-Bug: #1779335

.zuul.yaml

@@ -3,13 +3,13 @@
       jobs:
         - neutron-vpnaas-dsvm-functional-sswan
         - neutron-vpnaas-tempest
-        - openstack-tox-lower-constraints
+        - neutron-vpnaas-tox-lower-constraints
         - neutron-vpnaas-tempest-libreswan-centos
     gate:
       jobs:
         - neutron-vpnaas-dsvm-functional-sswan
         - neutron-vpnaas-tempest
-        - openstack-tox-lower-constraints
+        - neutron-vpnaas-tox-lower-constraints
     experimental:
       jobs:
         - neutron-vpnaas-dsvm-rally
@@ -85,3 +85,8 @@
       - ^neutron_vpnaas/tests/unit/.*$
       - ^releasenotes/.*$
 
+- job:
+    name: neutron-vpnaas-tox-lower-constraints
+    parent: openstack-tox-lower-constraints
+    required-projects:
+      - openstack/neutron

tox.ini

@@ -33,6 +33,22 @@ whitelist_externals =
   cp
   sudo
 
+[testenv:dev]
+# run locally (not in the gate) using editable mode
+# https://pip.pypa.io/en/stable/reference/pip_install/#editable-installs
+# note that order is important to ensure dependencies don't override
+commands =
+    pip install -q -e "git+https://git.openstack.org/openstack/neutron#egg=neutron"
+
+[testenv:py3-dev]
+basepython = python3
+commands =
+    {[testenv:dev]commands}
+    pip freeze
+    stestr run {posargs}
+whitelist_externals =
+    stestr
+
 [testenv:dsvm-functional]
 setenv =
   OS_TEST_PATH=./neutron_vpnaas/tests/functional/openswan