[3.13] gh-64308: Remove TestProgram from the unittest docs (GH-121675) (GH-121742)

(cherry picked from commit f6f4022)

Co-authored-by: Jan Musílek <jan.musilek@nic.cz>

Author: miss-islington

Doc/library/unittest.rst

@@ -2316,8 +2316,8 @@ Loading and running tests
    (see :ref:`Warning control <using-on-warnings>`),
    otherwise it will be set to ``'default'``.
 
-   Calling ``main`` actually returns an instance of the ``TestProgram`` class.
-   This stores the result of the tests run as the ``result`` attribute.
+   Calling ``main`` returns an object with the ``result`` attribute that contains
+   the result of the tests run as a :class:`unittest.TestResult`.
 
    .. versionchanged:: 3.1
       The *exit* parameter was added.