Fix breaking build due to new tox (#983)

Version 4 of Tox is currently breaking the build. Therefore, until
those issues are fixed, this change can revert to Tox 3.x.

Signed-off-by: Eric Brown <[email protected]>

Author: ericwb

tox.ini

@@ -1,7 +1,9 @@
 [tox]
-minversion = 2.0
+minversion = 3.2.0
 envlist = py37,pep8
 skipsdist = True
+requires =
+    tox<4
 
 [testenv]
 usedevelop = True
@@ -14,7 +16,7 @@ deps =
 commands =
     find bandit -type f -name "*.pyc" -delete
     stestr run {posargs}
-whitelist_externals =
+allowlist_externals =
     find
 passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY