Fix tox.ini with tox4

skipsdist setting breaks the docs environment

Change-Id: I191efbec1240ebadee2f030ea45fe09d03fde99e

tox.ini

@@ -1,10 +1,9 @@
 [tox]
 envlist = py39,pep8,pylint
 minversion = 3.18.0
-skipsdist = True
 
 [testenv]
-basepython = python3
+basepython = {env:TOX_PYTHON:python3}
 setenv = VIRTUAL_ENV={envdir}
          PYTHONWARNINGS=default::DeprecationWarning
          OS_LOG_CAPTURE={env:OS_LOG_CAPTURE:true}
@@ -15,7 +14,7 @@ deps = -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/
        -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 allowlist_externals =
-  sh
+  bash
   find
 commands =
   find . -type f -name "*.py[c|o]" -delete
@@ -84,7 +83,7 @@ setenv =
   OS_FAIL_ON_MISSING_DEPS=1
   OS_NEUTRON_PATH={env:OS_NEUTRON_PATH:/home/zuul/src/opendev.org/openstack/neutron}
 allowlist_externals =
-  sh
+  bash
   cp
   sudo
 commands =
@@ -99,11 +98,11 @@ commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasen
 commands =
   flake8
   doc8 {posargs}
-  {toxinidir}/tools/check_unit_test_structure.sh
+  bash {toxinidir}/tools/check_unit_test_structure.sh
   neutron-db-manage --subproject neutron-fwaas --database-connection sqlite:// check_migration
   {[testenv:genconfig]commands}
   {[testenv:genpolicy]commands}
-allowlist_externals = sh
+allowlist_externals = bash
 
 [testenv:cover]
 setenv = VIRTUAL_ENV={envdir}
@@ -182,7 +181,7 @@ import_exceptions = neutron_fwaas._i18n
 local-check-factory = neutron_lib.hacking.checks.factory
 
 [testenv:genconfig]
-commands = {toxinidir}/tools/generate_config_file_samples.sh
+commands = bash {toxinidir}/tools/generate_config_file_samples.sh
 
 [testenv:genpolicy]
 commands = oslopolicy-sample-generator --config-file=etc/oslo-policy-generator/policy.conf