Update tox from 3.11.1 to 3.12.1

pyup-bot · Ashwin Rajeev · commit ae5855e0b527 · 2019-06-03T15:55:46.000+05:30
diff --git a/requirements_dev.txt b/requirements_dev.txt
@@ -3,7 +3,7 @@ bumpversion==0.5.3
 wheel==0.33.4
 watchdog==0.9.0
 flake8==3.7.7
-tox==3.11.1
+tox==3.12.1
 coverage==4.5.3
 Sphinx==2.0.1
 twine==1.13.0
