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

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

setup.py

@@ -6,7 +6,7 @@ install_requires = ['execnet>=1.1', 'pytest>=3.0.0', 'pytest-forked']
 setup(
     name="pytest-xdist",
     use_scm_version={'write_to': 'xdist/_version.py'},
-    description='py.test xdist plugin for distributed testing'
+    description='pytest xdist plugin for distributed testing'
                 ' and loop-on-failing modes',
     long_description=open('README.rst').read(),
     license='MIT',