Add import of `unittest.mock.Mock` in documentation (#102346)

Doc/library/unittest.mock.rst

@@ -72,6 +72,7 @@ available, and then make assertions about how they have been used:
 :attr:`side_effect` allows you to perform side effects, including raising an
 exception when a mock is called:
 
+   >>> from unittest.mock import Mock
    >>> mock = Mock(side_effect=KeyError('foo'))
    >>> mock()
    Traceback (most recent call last):