fix: derive from TestCase

Author: blueyed

mixin.py

@@ -1,3 +1,4 @@
 class MyMixin:
+    @classmethod
     def setUpClass(cls):
         print('MyMixin.setUpClass', cls)

mod1/__init__.py

@@ -1,6 +1,8 @@
+from unittest import TestCase
+
 from mixin import MyMixin
 
 
-class FirstTestCase(MyMixin):
+class FirstTestCase(MyMixin, TestCase):
     def test_1st(self):
         print("test_1st", self)