Python unittest - setUpClass() дает мне проблемы - почему я не могу так наследовать?

У меня есть код unittest, например:

import unittest

class MyUnitTest(unittest.TestCase):
    def setUpClass(self):
        do_something_expensive_for_all_sets_of_tests()

class MyFirstSetOfTests(MyUnitTest):
    def setUpClass(self):
        super(MyFirstSetOfTests, self).setUpClass()
        do_something_expensive_for_just_these_first_tests()

    def test_one(self):
        ...

    def test_two(self):
        ...

class MySecondSetOfTests(MyUnitTest):
    def setUpClass(self):
        super(MySecondSetOfTests, self).setUpClass()
        do_something_expensive_for_just_these_second_tests()

    def test_one(self):
        ...

    def test_two(self):
        ...

if __name__ == '__main__':
    unittest.main()

Когда я пытаюсь запустить этот код, я получаю сообщение об ошибке:

======================================================================
ERROR: setUpClass (__main__.MyFirstSetOfTests)
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TypeError: unbound method setUpClass() must be called with MyFirstSetOfTests instance as first argument (got nothing instead)

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