add pytest to test env requirements and suggest it in contributing docs

CONTRIBUTING.rst

@@ -136,11 +136,11 @@ Install dependencies::
 
 To run the Python tests, use::
 
-    nosetests
+    pytest
 
 If you want coverage statistics as well, you can run::
 
-    nosetests --with-coverage --cover-package=notebook notebook
+    py.test --cov notebook -v --pyargs notebook
 
 JavaScript Tests
 ^^^^^^^^^^^^^^^^

setup.py

@@ -97,7 +97,7 @@ for more information.
         ':python_version == "2.7"': ['ipaddress'],
         'test:python_version == "2.7"': ['mock'],
         'test': ['nose', 'coverage', 'requests', 'nose_warnings_filters',
-                 'nbval', 'nose-exclude', 'selenium'],
+                 'nbval', 'nose-exclude', 'selenium', 'pytest', 'pytest-cov'],
         'test:sys_platform == "win32"': ['nose-exclude'],
     },
     entry_points = {