Fix tox warning regarding black

cburgdorf · cburgdorf · commit 7128bb073d6c · 2019-11-05T00:14:16.000+09:00
diff --git a/tox.ini b/tox.ini
@@ -42,3 +42,4 @@ commands=
     isort --recursive --check-only --diff {toxinidir}/dopple {toxinidir}/tests
     pydocstyle {toxinidir}/dopple {toxinidir}/tests
     black --check --diff {toxinidir}/dopple {toxinidir}/tests
+whitelist_externals=black
