A warning is now issued when assertions are made for ``None``.

This is a common source of confusion among new users, which write::

    assert mocked_object.assert_called_with(3, 4, 5, key='value')

When they should write::

    mocked_object.assert_called_with(3, 4, 5, key='value')

Because the ``assert_called_with`` method of mock objects already executes an assertion.

This warning will not be issued when ``None`` is explicitly checked. An assertion like::

    assert variable is None

will not issue the warning.