Adjust tox.ini for tox4

OpenStack CI now uses tox4 by default.

- Drop "skipsdist = True".
  When skipsdist=True is specified, a target application is not
  installed into a tox env, but there seems no reason to do so.
  In various other OpenStack projects, a target applicatin needs
  to be installed into a tox vnev, for example, in docs and/or
  linter jobs and it causes failures with tox4. So dropping it
  would avoid potential future failures.
- Add ignore_basepython_conflict = True to tox.ini
  With latest tox (>4.0.15) python version conflicted with base
  python. Adding ignore_basepython_conflict = True solves this
  issue.
- Replace deprecated whitelist_externals with allowlist_externals.

Change-Id: Iead95c0779f9705cf096d1ac3038a16aa62f1678

tox.ini

@@ -1,7 +1,10 @@
 [tox]
 envlist = py3,pep8,,py36-local,eslint,karma-local,docs-local,releasenotes,lower-constraints-local
 minversion = 2.3.2
-skipsdist = True
+# Automatic envs (pyXX) will only use the python version appropriate to that
+# env and ignore basepython inherited from [testenv] if we set
+# ignore_basepython_conflict.
+ignore_basepython_conflict = True
 
 [testenv]
 basepython = python3
@@ -46,7 +49,7 @@ commands =
   {[testenv]commands}
 
 [testenv:eslint]
-whitelist_externals =
+allowlist_externals =
   npm
 commands =
   npm install
@@ -56,15 +59,15 @@ commands =
 # from master branch into py35 environment for testing javascripts.
 # Horizon from master is needed to be cloned into ../horizon on both local and CI.
 [testenv:karma]
-whitelist_externals =
-  {[testenv:eslint]whitelist_externals}
+allowlist_externals =
+  {[testenv:eslint]allowlist_externals}
 commands =
   npm install
   npm run test
 
 [testenv:karma-local]
-whitelist_externals =
-  {[testenv:eslint]whitelist_externals}
+allowlist_externals =
+  {[testenv:eslint]allowlist_externals}
 commands =
   {[testenv:karma]commands}
 
@@ -92,7 +95,7 @@ commands =
   sphinx-build -a -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:debug]
-whitelist_externals = oslo_debug_helper
+allowlist_externals = oslo_debug_helper
 commands = oslo_debug_helper -t senlin_dashboard/test {posargs}
 
 [testenv:makemessages]