Add an rq.filter to ensure pytest < 5

This is required as pytest >= 5 is Python 3 only and Hyper-h2 supports
Python 2.

Author: pgjones

test_requirements.txt

@@ -1,4 +1,4 @@
-pytest==4.6.2
+pytest==4.6.2  # rq.filter < 5
 pytest-cov==2.7.1
 coverage==4.5.3
 pytest-xdist==1.29.0