Update pytest from 3.6.4 to 3.7.1 (#115)

Author: pyup-bot

axe_selenium_python/tests/requirements/tests.txt

@@ -1,4 +1,4 @@
-pytest==3.6.4
+pytest==3.7.1
 selenium==3.14.0
 pytest-selenium==1.13.0
 pytest-html==1.19.0