DOC: Update documentation for TestCase usage

tm.TestCase no longer follows the nosetest idiom,
so it is here to stay, so update the documentation
to say that we are using it still.

Closes gh-15990.

Author: gfyoung

doc/source/contributing.rst

@@ -632,14 +632,6 @@ framework that will facilitate testing and developing. Thus, instead of writing
     def test_really_cool_feature():
         ....
 
-Sometimes, it does make sense to bundle test functions together into a single class, either because the test file is testing multiple functions from a single module, and
-using test classes allows for better organization. However, instead of inheriting from ``tm.TestCase``, we should just inherit from ``object``:
-
-.. code-block:: python
-
-    class TestReallyCoolFeature(object):
-        ....
-
 Using ``pytest``
 ~~~~~~~~~~~~~~~~
 

pandas/tests/test_algos.py

@@ -282,7 +282,7 @@ def test_complex_sorting(self):
         # gh 12666 - check no segfault
         # Test not valid numpy versions older than 1.11
         if pd._np_version_under1p11:
-            self.skipTest("Test valid only for numpy 1.11+")
+            pytest.skip("Test valid only for numpy 1.11+")
 
         x17 = np.array([complex(i) for i in range(17)], dtype=object)