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

Author: munkm

CONTRIBUTING.rst

@@ -65,11 +65,11 @@ 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=jupyter_server jupyter_server
+    py.test --cov notebook -v --pyargs jupyter_server
 
 Building the Documentation
 --------------------------

setup.py

@@ -92,7 +92,7 @@
     ],
     extras_require = {
         'test': ['nose', 'coverage', 'requests', 'nose_warnings_filters',
-                 'nbval', 'nose-exclude', 'selenium'],
+                 'nbval', 'nose-exclude', 'selenium', 'pytest', 'pytest-cov'],
         'test:sys_platform == "win32"': ['nose-exclude'],
     },
     python_requires='>=3.5',