[ussuri][goal] Updates for python 2.7 drop

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

openstack-ansible repos need few updates on requirements
and tox file.

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

Depends-On: https://review.opendev.org/711015/
Change-Id: I49e749724b4e4b7c2cb7eab188bc676a86a47ee1

tox.ini

@@ -1,10 +1,11 @@
 [tox]
-minversion = 2.0
+minversion = 3.1
 skipsdist = True
 envlist = docs,linters,functional
-
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 install_command =
     pip install -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
@@ -34,7 +35,6 @@ setenv =
 
 
 [testenv:docs]
-basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands =
     bash -c "rm -rf doc/build"
@@ -43,7 +43,6 @@ commands =
 
 
 [testenv:pdf-docs]
-basepython = python3
 deps = {[testenv:docs]deps}
 whitelist_externals =
   make
@@ -58,7 +57,6 @@ extensions = .rst
 
 
 [testenv:releasenotes]
-basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands =
     sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
@@ -66,13 +64,11 @@ commands =
 
 # environment used by the -infra templated docs job
 [testenv:venv]
-basepython = python3
 commands =
     {posargs}
 
 
 [testenv:pep8]
-basepython = python3
 commands =
     bash -c "{toxinidir}/tests/common/test-pep8.sh"
 
@@ -85,7 +81,6 @@ ignore=F403
 
 
 [testenv:bashate]
-basepython = python3
 commands =
     bash -c "{toxinidir}/tests/common/test-bashate.sh"
 
@@ -102,13 +97,11 @@ commands =
 
 
 [testenv:ansible-syntax]
-basepython = python3
 commands =
     bash -c "{toxinidir}/tests/common/test-ansible-syntax.sh"
 
 
 [testenv:ansible-lint]
-basepython = python3
 commands =
     bash -c "{toxinidir}/tests/common/test-ansible-lint.sh"
 
@@ -119,7 +112,6 @@ commands =
 
 
 [testenv:linters]
-basepython = python3
 commands =
     bash -c "{toxinidir}/tests/common/test-ansible-env-prep.sh"
     {[testenv:pep8]commands}