Fix tox4 error

tox.ini started failing with Tox4 which had some
incompatible changes. One is allowlist_externals which now needs to be
explicitly set for each testenv.

Other failure is due to the skipsdist = True.

Fixing tox.ini for tox4 changes.

Change-Id: I7ef284a07866d99cbc34b391295efbae261af7c6

tox.ini

@@ -2,7 +2,6 @@
 minversion = 3.18.0
 envlist = py3,flake8
 ignore_basepython_conflict = True
-skipsdist = True
 
 [testenv]
 basepython = python3
@@ -41,6 +40,8 @@ allowlist_externals =
   rm
 
 [testenv:docs]
+allowlist_externals =
+  rm
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/requirements.txt