Add an rq.filter to ensure pytest < 5
Add an rq.filter to ensure pytest < 5

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

Kriechi/hyper-h2,Kriechi/hyper-h2,python-hyper/hyper-h2,python-hyper/hyper-h2