DOC: Add doc explaining TestCase usage under pytest

Author: gfyoung

pandas/util/testing.py

@@ -83,6 +83,14 @@ def reset_testing_mode():
 
 
 class TestCase(unittest.TestCase):
+    """
+    The test case class that we originally used when using the
+    nosetests framework. Under the new pytest framework, we are
+    moving away from this class.
+
+    Do not create new test classes derived from this one. Rather,
+    they should inherit from object directly.
+    """
 
     @classmethod
     def setUpClass(cls):