Update pytest from 3.8.2 to 3.9.1 (#148)

Author: pyup-bot

axe_selenium_python/tests/requirements/tests.txt

@@ -1,4 +1,4 @@
-pytest==3.8.2
+pytest==3.9.1
 selenium==3.14.1
 pytest-html==1.19.0
 pytest-mock==1.10.0