unittest documentation: Spell pytest without the dot (GH-9820)

Referring to ``pytest`` as ``py.test`` is deprecated.

Doc/library/unittest.rst

@@ -56,7 +56,7 @@ test runner
       Kent Beck's original paper on testing frameworks using the pattern shared
       by :mod:`unittest`.
 
-   `Nose <https://nose.readthedocs.io/>`_ and `py.test <https://docs.pytest.org/>`_
+   `Nose <https://nose.readthedocs.io/>`_ and `pytest <https://docs.pytest.org/>`_
       Third-party unittest frameworks with a lighter-weight syntax for writing
       tests.  For example, ``assert func(10) == 42``.