Remove doc reference to unmaitained Nose package (GH-16849)

The Nose package is no longer maintained.

Doc/library/unittest.rst

@@ -56,8 +56,8 @@ test runner
       Kent Beck's original paper on testing frameworks using the pattern shared
       by :mod:`unittest`.
 
-   `Nose <https://nose.readthedocs.io/>`_ and `pytest <https://docs.pytest.org/>`_
-      Third-party unittest frameworks with a lighter-weight syntax for writing
+   `pytest <https://docs.pytest.org/>`_
+      Third-party unittest framework with a lighter-weight syntax for writing
       tests.  For example, ``assert func(10) == 42``.
 
    `The Python Testing Tools Taxonomy <https://wiki.python.org/moin/PythonTestingToolsTaxonomy>`_