fix: test case

Author: irtazaakram

xmodule/tests/test_annotatable_block.py

@@ -1,8 +1,7 @@
 """Annotatable block tests"""
 
 
-import unittest
-
+from django.test import TestCase
 from lxml import etree
 from opaque_keys.edx.locator import BlockUsageLocator, CourseLocator
 from xblock.field_data import DictFieldData
@@ -13,7 +12,7 @@
 from . import get_test_system
 
 
-class AnnotatableBlockTestCase(unittest.TestCase):  # lint-amnesty, pylint: disable=missing-class-docstring
+class AnnotatableBlockTestCase(TestCase):  # lint-amnesty, pylint: disable=missing-class-docstring
     sample_xml = '''
         <annotatable display_name="Iliad">
             <instructions>Read the text.</instructions>