Update tox and add lower-constraints

Change-Id: Ia68b6e7252a2c971d059962b8302661e272ec24d
Story: #2003734
Task: #26399
Signed-off-by: Trinh Nguyen <dangtrinhnt@gmail.com>

lower-constraints.txt

@@ -0,0 +1,10 @@
+coverage==4.0 # Apache-2.0
+hacking==0.12.0 # Apache-2.0
+openstackdocstheme==1.18.1  # Apache-2.0
+oslotest==1.10.0 # Apache-2.0
+pbr==2.0 # Apache-2.0
+python-subunit==0.0.18 # Apache-2.0/BSD
+reno==2.5.0 # Apache-2.0
+sphinx==1.6.2 # BSD
+stestr==1.0.0 # Apache-2.0
+testtools==1.4.0 # MIT

tox.ini

@@ -1,27 +1,33 @@
 [tox]
 minversion = 2.0
-envlist = py35,pep8
+envlist = py36,py35,py27,pep8,docs
 skipsdist = True
 
 [testenv]
 usedevelop = True
-install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
+install_command = pip install -U {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
    PYTHONWARNINGS=default::DeprecationWarning
    OS_STDOUT_CAPTURE=1
    OS_STDERR_CAPTURE=1
    OS_TEST_TIMEOUT=60
-deps = -r{toxinidir}/test-requirements.txt
+deps =
+  -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
+  -r{toxinidir}/requirements.txt
+  -r{toxinidir}/test-requirements.txt
 commands = stestr run {posargs}
 
 [testenv:pep8]
+basepython = python3
 commands = flake8 {posargs}
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 setenv =
     VIRTUAL_ENV={envdir}
     PYTHON=coverage run --source fenixclient --parallel-mode
@@ -32,17 +38,27 @@ commands =
     coverage xml -o cover/coverage.xml
 
 [testenv:docs]
+basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:releasenotes]
+basepython = python3
 deps = {[testenv:docs]deps}
 commands =
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:debug]
+basepython = python3
 commands = oslo_debug_helper {posargs}
 
+[testenv:lower-constraints]
+basepython = python3
+deps =
+  -c{toxinidir}/lower-constraints.txt
+  -r{toxinidir}/test-requirements.txt
+  -r{toxinidir}/requirements.txt
+
 [flake8]
 # E123, E125 skipped as they are invalid PEP-8.