remove pytest-runner, other code supporting python 2.x
remove pytest-runner, other code supporting python 2.x

Parsimonious no longer supports python 2.x and this removes code from setup.py that supported python 2.x patterns and trigger warnings in python 3.x. In a post PEP 517 world, tox manages the test dependencies.

erikrose/parsimonious,lucaswiman/parsimonious