Drop environments for old python versions

Python 3.6, 3.7 and 3.8 support were already removed.

Use the unversioned target so that we don't have to update it when
supported python versions are updated.

Change-Id: I4306ef2285678fe66a5908fc4ea240bc1511d145

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.18.0
-envlist = py39,pep8
+envlist = py3,pep8
 ignore_basepython_conflict = true
 
 [testenv]
@@ -8,6 +8,7 @@ basepython = python3
 usedevelop = True
 setenv =
    VIRTUAL_ENV={envdir}
+   DJANGO_SETTINGS_MODULE=manila_ui.tests.settings
 deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
@@ -16,17 +17,7 @@ allowlist_externals =
 commands =
   /bin/bash run_tests.sh -N --no-pep8 {posargs}
 
-[testenv:py36]
-setenv = DJANGO_SETTINGS_MODULE=manila_ui.tests.settings
-
-[testenv:py37]
-setenv = DJANGO_SETTINGS_MODULE=manila_ui.tests.settings
-
-[testenv:py38]
-setenv = DJANGO_SETTINGS_MODULE=manila_ui.tests.settings
-
 [testenv:py3-dev]
-setenv = DJANGO_SETTINGS_MODULE=manila_ui.tests.settings
 commands =
   pip install {env:HORIZON_REPO:git+https://opendev.org/openstack/horizon}
   {[testenv]commands}