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

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

qa-specs adjust the tox update for basepython to common env.

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: I4e3dcbbe657caed5d3e71cd0a5b80d641616df2a

tox.ini

@@ -1,20 +1,19 @@
 [tox]
-minversion = 2.0
+minversion = 3.1.1
 envlist = docs
 skipsdist = True
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 setenv = VIRTUAL_ENV={envdir}
-install_command = pip install -U {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:docs]
-basepython = python3
 commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:spelling]