Use pytest instead of py.test per upstream recommendation, #dropthedot (#183)

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

Author: laurenyu

tox.ini

@@ -54,7 +54,7 @@ passenv =
 # {posargs} can be passed in by additional arguments specified when invoking tox.
 # Can be used to specify which tests to run, e.g.: tox -- -s
 commands =
-    coverage run --source sagemaker -m py.test {posargs}
+    coverage run --source sagemaker -m pytest {posargs}
     {env:IGNORE_COVERAGE:} coverage report --fail-under=90 --omit */tensorflow/tensorflow_serving/*
 deps =
     pytest