Fix tox4 error

tox.ini started failing with Tox4 which had some
incompatible changes. One of them is changing
whitelist to allowlist. Other failure is due to
the skipsdist = True.

Fixing tox.ini for tox4 changes.

Change-Id: I113bb6dd90efe0f9b272a76939947bd913fe707f

tox.ini

@@ -1,7 +1,6 @@
 [tox]
 minversion = 3.1.1
 envlist = py37,pep8
-skipsdist = True
 ignore_basepython_conflict = True
 
 [testenv]
@@ -17,7 +16,7 @@ deps =
        -r{toxinidir}/test-requirements.txt
 commands = find . -type f -name "*.pyc" -delete
            stestr run {posargs}
-whitelist_externals = find
+Allowlist_externals = find
 
 [testenv:pep8]
 commands = flake8