Define python3 as basepython for Tempest tox env

While OpenStack is in transition to drop py2, many
lib or dependency of Tempest or its plugins cannot
be installed on python 2.7.

To install Tempest on py3 evnv, let's change the basepython
as python 3 for tox env.

Defining basepython as python3.6 for all venv using
.tox/tempest.

Depends-On: https://review.opendev.org/#/c/703476/
Depends-On: https://review.opendev.org/#/c/703679/

Closes-Bug: 1860033
Change-Id: If3ff71dbe636ef98072b15f6599e6e641c222f30

tox.ini

@@ -1,9 +1,11 @@
 [tox]
 envlist = pep8,py36,py37,py27,bashate,pip-check-reqs
-minversion = 2.3.1
+minversion = 3.1.1
 skipsdist = True
+ignore_basepython_conflict = True
 
 [tempestenv]
+basepython = python3.6
 sitepackages = False
 setenv =
     VIRTUAL_ENV={envdir}
@@ -13,6 +15,7 @@ deps =
     -r{toxinidir}/requirements.txt
 
 [testenv]
+basepython = python3
 setenv =
     VIRTUAL_ENV={envdir}
     OS_LOG_CAPTURE=1
@@ -49,12 +52,12 @@ commands =
   coverage report
 
 [testenv:debug]
-basepython = python3
 commands = oslo_debug_helper -t tempest/tests {posargs}
 
 [testenv:all]
 envdir = .tox/tempest
 sitepackages = {[tempestenv]sitepackages}
+basepython = {[tempestenv]basepython}
 # 'all' includes slow tests
 setenv =
     {[tempestenv]setenv}
@@ -77,6 +80,7 @@ sitepackages = True
 setenv =
     {[tempestenv]setenv}
     OS_TEST_TIMEOUT={env:OS_TEST_TIMEOUT:1200}
+basepython = {[tempestenv]basepython}
 deps = {[tempestenv]deps}
 commands =
     echo "WARNING: The all-plugin env is deprecated and will be removed"
@@ -90,6 +94,7 @@ sitepackages = True
 setenv =
     {[tempestenv]setenv}
     OS_TEST_TIMEOUT={env:OS_TEST_TIMEOUT:1200}
+basepython = {[tempestenv]basepython}
 deps = {[tempestenv]deps}
 commands =
     find . -type f -name "*.pyc" -delete
@@ -98,6 +103,7 @@ commands =
 [testenv:full]
 envdir = .tox/tempest
 sitepackages = {[tempestenv]sitepackages}
+basepython = {[tempestenv]basepython}
 setenv = {[tempestenv]setenv}
 deps = {[tempestenv]deps}
 # The regex below is used to select which tests to run and exclude the slow tag:
@@ -111,6 +117,7 @@ commands =
 [testenv:full-parallel]
 envdir = .tox/tempest
 sitepackages = {[tempestenv]sitepackages}
+basepython = {[tempestenv]basepython}
 setenv = {[tempestenv]setenv}
 deps = {[tempestenv]deps}
 # The regex below is used to select all tempest scenario and including the non slow api tests
@@ -121,6 +128,7 @@ commands =
 [testenv:integrated-network]
 envdir = .tox/tempest
 sitepackages = {[tempestenv]sitepackages}
+basepython = {[tempestenv]basepython}
 setenv = {[tempestenv]setenv}
 deps = {[tempestenv]deps}
 # The regex below is used to select which tests to run and exclude the slow tag and
@@ -133,6 +141,7 @@ commands =
 [testenv:integrated-compute]
 envdir = .tox/tempest
 sitepackages = {[tempestenv]sitepackages}
+basepython = {[tempestenv]basepython}
 setenv = {[tempestenv]setenv}
 deps = {[tempestenv]deps}
 # The regex below is used to select which tests to run and exclude the slow tag and
@@ -145,6 +154,7 @@ commands =
 [testenv:integrated-placement]
 envdir = .tox/tempest
 sitepackages = {[tempestenv]sitepackages}
+basepython = {[tempestenv]basepython}
 setenv = {[tempestenv]setenv}
 deps = {[tempestenv]deps}
 # The regex below is used to select which tests to run and exclude the slow tag and
@@ -157,6 +167,7 @@ commands =
 [testenv:integrated-storage]
 envdir = .tox/tempest
 sitepackages = {[tempestenv]sitepackages}
+basepython = {[tempestenv]basepython}
 setenv = {[tempestenv]setenv}
 deps = {[tempestenv]deps}
 # The regex below is used to select which tests to run and exclude the slow tag and
@@ -169,6 +180,7 @@ commands =
 [testenv:integrated-object-storage]
 envdir = .tox/tempest
 sitepackages = {[tempestenv]sitepackages}
+basepython = {[tempestenv]basepython}
 setenv = {[tempestenv]setenv}
 deps = {[tempestenv]deps}
 # The regex below is used to select which tests to run and exclude the slow tag and
@@ -181,6 +193,7 @@ commands =
 [testenv:full-serial]
 envdir = .tox/tempest
 sitepackages = {[tempestenv]sitepackages}
+basepython = {[tempestenv]basepython}
 setenv = {[tempestenv]setenv}
 deps = {[tempestenv]deps}
 # The regex below is used to select which tests to run and exclude the slow tag:
@@ -193,6 +206,7 @@ commands =
 [testenv:scenario]
 envdir = .tox/tempest
 sitepackages = {[tempestenv]sitepackages}
+basepython = {[tempestenv]basepython}
 setenv = {[tempestenv]setenv}
 deps = {[tempestenv]deps}
 # The regex below is used to select all scenario tests
@@ -203,6 +217,7 @@ commands =
 [testenv:smoke]
 envdir = .tox/tempest
 sitepackages = {[tempestenv]sitepackages}
+basepython = {[tempestenv]basepython}
 setenv = {[tempestenv]setenv}
 deps = {[tempestenv]deps}
 commands =
@@ -212,6 +227,7 @@ commands =
 [testenv:smoke-serial]
 envdir = .tox/tempest
 sitepackages = {[tempestenv]sitepackages}
+basepython = {[tempestenv]basepython}
 setenv = {[tempestenv]setenv}
 deps = {[tempestenv]deps}
 # This is still serial because neutron doesn't work with parallel. See:
@@ -224,6 +240,7 @@ commands =
 [testenv:slow-serial]
 envdir = .tox/tempest
 sitepackages = {[tempestenv]sitepackages}
+basepython = {[tempestenv]basepython}
 setenv = {[tempestenv]setenv}
 deps = {[tempestenv]deps}
 # The regex below is used to select the slow tagged tests to run serially:
@@ -234,6 +251,7 @@ commands =
 [testenv:ipv6-only]
 envdir = .tox/tempest
 sitepackages = {[tempestenv]sitepackages}
+basepython = {[tempestenv]basepython}
 setenv = {[tempestenv]setenv}
 deps = {[tempestenv]deps}
 # Run only smoke and ipv6 tests. This env is used to tests
@@ -253,12 +271,12 @@ commands = {posargs}
 [testenv:venv-tempest]
 envdir = .tox/tempest
 sitepackages = {[tempestenv]sitepackages}
+basepython = {[tempestenv]basepython}
 setenv = {[tempestenv]setenv}
 deps = {[tempestenv]deps}
 commands = {posargs}
 
 [testenv:docs]
-basepython = python3
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/requirements.txt
@@ -269,7 +287,6 @@ commands =
 whitelist_externals = rm
 
 [testenv:pdf-docs]
-basepython = python3
 deps = {[testenv:docs]deps}
 whitelist_externals =
    make
@@ -281,7 +298,6 @@ commands =
 deps =
     -r{toxinidir}/test-requirements.txt
     autopep8
-basepython = python3
 commands =
     autopep8 --exit-code --max-line-length=79 --experimental --diff -r tempest setup.py
     flake8 {posargs}
@@ -289,7 +305,6 @@ commands =
 
 [testenv:autopep8]
 deps = autopep8
-basepython = python3
 commands =
     {toxinidir}/tools/format.sh
 
@@ -313,7 +328,6 @@ enable-extensions = H106,H203,H904
 import-order-style = pep8
 
 [testenv:releasenotes]
-basepython = python3
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/requirements.txt
@@ -325,7 +339,6 @@ commands =
 whitelist_externals = rm
 
 [testenv:bashate]
-basepython = python3
 # if you want to test out some changes you have made to bashate
 # against tempest, just set BASHATE_INSTALL_PATH=/path/... to your
 # modified bashate tree
@@ -360,7 +373,6 @@ commands = bindep test
 
 [testenv:plugin-sanity-check]
 # perform tempest plugin sanity
-basepython = python3
 whitelist_externals = bash
 commands =
   bash tools/tempest-plugin-sanity.sh