[ussuri][goal] Updates for python 2.7 drop

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

Only update required is in tox to define common baspython as py3 os that
all tox env including [testenv:functional] will use py3 instead of py2.

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: I4066203c7eaff4b5b0e5d3e2e659d68c7f1bae25

tox.ini

@@ -1,9 +1,11 @@
 [tox]
-minversion = 2.0
+minversion = 3.1
 envlist = docs
 skipsdist = True
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 install_command =
   pip install -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt} {opts} {packages}
 setenv =
@@ -18,11 +20,9 @@ whitelist_externals =
   bash
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:docs]
-basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands =
   bash -c "rm -rf doc/build"
@@ -33,7 +33,6 @@ commands =
 extensions = .rst
 
 [testenv:releasenotes]
-basepython = python3
 whitelist_externals =
   rm
 deps = {[testenv:docs]deps}