Use pytest instead of py.test per upstream recommendation, #dropthedot (

#255)

http://blog.pytest.org/2016/whats-new-in-pytest-30/
https://twitter.com/hashtag/dropthedot

system_tests/app_engine_test_app/main.py

@@ -15,8 +15,8 @@
 """App Engine standard application that runs basic system tests for
 google.auth.app_engine.
 
-This application has to run tests manually instead of using py.test because
-py.test currently doesn't work on App Engine standard.
+This application has to run tests manually instead of using pytest because
+pytest currently doesn't work on App Engine standard.
 """
 
 import contextlib

tox.ini

@@ -16,12 +16,12 @@ deps =
   cryptography
   grpcio; platform_python_implementation != 'PyPy'
 commands =
-  py.test --cov=google.auth --cov=google.oauth2 --cov=tests {posargs:tests}
+  pytest --cov=google.auth --cov=google.oauth2 --cov=tests {posargs:tests}
 
 [testenv:cover]
 basepython = python3.6
 commands =
-  py.test --cov=google.auth --cov=google.oauth2 --cov=tests --cov-report= tests
+  pytest --cov=google.auth --cov=google.oauth2 --cov=tests --cov-report= tests
   coverage report --show-missing --fail-under=100
 deps =
   {[testenv]deps}