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.
- whitelist_externals replaced with allowlist_externals since
  tox 3.18.0
  whitelist_externals option has been deprecated in
  favor of the new allowlist_externals option[1].

[1] https://github.com/tox-dev/tox/blob/legacy/docs/changelog.rst#v3180-2020-07-23

Change-Id: I478545d83231119fda0661f800246be7680e5190

tox.ini

@@ -1,7 +1,6 @@
 [tox]
 minversion = 3.1.1
 envlist = py38,py39,pep8
-skipsdist = True
 ignore_basepython_conflict = True
 
 [testenv]
@@ -10,7 +9,7 @@ usedevelop = True
 deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
-whitelist_externals = /bin/bash
+allowlist_externals = /bin/bash
 commands =
   {envpython} {toxinidir}/manage.py test --settings=sahara_dashboard.test.settings --exclude-tag integration {posargs}