Fix tox4 error

tox.ini started failing with tox4 which had some
incompatible changes. One of them is to allow
external command to run by adding them into allowlist_externals
and make passenv in new lines.

Fixing tox.ini for tox4 changes.

Change-Id: Id8d5c7bed75b6dc813203d2e814642855d645b6c

tox.ini

@@ -1,7 +1,6 @@
 [tox]
 minversion = 3.18.0
 envlist = py3,pep8
-skipsdist = True
 ignore_basepython_conflict = True
 
 [testenv]
@@ -10,7 +9,13 @@ usedevelop = True
 setenv = VIRTUAL_ENV={envdir}
          PYTHONDONTWRITEBYTECODE = 1
          PYTHONWARNINGS=default::DeprecationWarning
-passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
+passenv =
+  http_proxy
+  HTTP_PROXY
+  https_proxy
+  HTTPS_PROXY
+  no_proxy
+  NO_PROXY
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/test-requirements.txt