Install pytest by default in pylint runs

# <2.6 version bound put in place out of an abundance of cautiousness, no particular reason

# 2020-07-15 AK

$PY_EXE -m pip install "pylint<2.6" PyYAML

# pytest is being installed since test_*.py modules may import pytest, which

# pylint may inspect.

$PY_EXE -m pip install "pylint<2.6" PyYAML pytest

PYLINT_RUNNER_ARGS="--yaml-rcfile=.pylintrc.yml"