[ussuri][goal] Drop python 2.7 support and testing

OpenStack is dropping the py2.7 support in ussuri cycle.

sahara-image-elements is ready with python 3 and ok to drop the
python 2.7 support.

Complete discussion & schedule can be found in
- http://lists.openstack.org/pipermail/openstack-discuss/2019-October/010142.html
- https://etherpad.openstack.org/p/drop-python2-support

Ussuri Communtiy-wide goal:
https://governance.openstack.org/tc/goals/selected/ussuri/drop-py27.html

Change-Id: I5680fecb667038e3fcededaa89df815c07a913c7

tox.ini

@@ -4,6 +4,7 @@ minversion = 2.0
 skipsdist = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
 setenv =
@@ -18,7 +19,6 @@ whitelist_externals = bash
 commands = bash -c "find {toxinidir} -not -wholename \*.tox/\* -and \( -name \*.sh -or -wholename \*.d/\* -and -not -name \*.md -and -not -name \*.rst -and -not -name \*.py \) -print0 | xargs -0 bashate -v"
 
 [testenv:pep8]
-basepython = python3
 setenv =
     ELEMENTS_DIR=elements
 commands =
@@ -39,7 +39,6 @@ builtins = _
 exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,tools
 
 [testenv:lower-constraints]
-basepython = python3
 deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt