Use only py3 as base

Depends-On: https://review.opendev.org/#/c/700512/
Change-Id: If5df9893cc248efd8b8369b4eaaaca352bdf95d7

tox.ini

@@ -1,9 +1,11 @@
 [tox]
-minversion = 2.3.1
+minversion = 3.1.1
 envlist = py37,pep8
 skipsdist = True
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 install_command = pip install -U -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt} {opts} {packages}
 whitelist_externals = find
@@ -18,7 +20,6 @@ commands =
 passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
 
 [testenv:pep8]
-basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
        -c{toxinidir}/lower-constraints.txt
        -r{toxinidir}/test-requirements.txt
@@ -26,19 +27,15 @@ deps = -r{toxinidir}/doc/requirements.txt
 commands = flake8
 
 [testenv:genconfig]
-basepython = python3
 commands = oslo-config-generator --config-file=etc/vitrage/vitrage-config-generator.conf
 
 [testenv:genpolicy]
-basepython = python3
 commands = oslopolicy-sample-generator --config-file=etc/vitrage/vitrage-policy-generator.conf
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:cover]
-basepython = python3
 setenv =
     PYTHON=coverage run --source $project --parallel-mode
 commands =
@@ -49,7 +46,6 @@ commands =
     coverage report
 
 [testenv:docs]
-basepython = python3
 deps =
     -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
         -r{toxinidir}/requirements.txt
@@ -57,7 +53,6 @@ deps =
 commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:debug]
-basepython = python3
 commands = oslo_debug_helper {posargs}
 
 [flake8]
@@ -77,7 +72,6 @@ local-check-factory = vitrage.hacking.checks.factory
 import_exceptions = vitrage.i18n
 
 [testenv:releasenotes]
-basepython = python3
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
   -r{toxinidir}/requirements.txt
@@ -85,7 +79,6 @@ deps =
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:lower-constraints]
-basepython = python3
 deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt