tox.ini: use pytest-randomly to randomize test execution

https://github.com/pytest-dev/pytest-randomly
diff --git a/tox.ini b/tox.ini
index 2e8d9ee..6a01501 100644
--- a/tox.ini
+++ b/tox.ini
@@ -6,6 +6,7 @@
 deps =
     cov: coverage>=4.3
     pytest
+    pytest-randomly
     -rrequirements.txt
 extras =
     ufo