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
diff --git a/tox.ini b/tox.ini
index be124e6..1c52c4b 100644
--- a/tox.ini
+++ b/tox.ini
@@ -16,12 +16,12 @@
   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}