Update pytest for most Python versions. (#318)

Author: benjaminp

.travis.yml

@@ -15,7 +15,7 @@ install:
 - pip install --upgrade --force-reinstall "setuptools; python_version != '3.3'" "setuptools < 40; python_version == '3.3'"
 - pip uninstall --yes six || true
 - pip install --upgrade --force-reinstall --ignore-installed -e .
-- pip install pytest==2.9.2 typing
+- pip install "pytest==4.6.9; python_version != '3.3'" "pytest==2.9.2; python_version == '3.3'" typing
 - &py_pkg_list pip list --format=columns || pip list
 script:
 - py.test