diff --git a/IPython/nbformat/v3/tests/formattest.py b/IPython/nbformat/v3/tests/formattest.py
index 1b6088c..d19725c 100644
--- a/IPython/nbformat/v3/tests/formattest.py
+++ b/IPython/nbformat/v3/tests/formattest.py
@@ -6,8 +6,6 @@ import tempfile
 
 pjoin = os.path.join
 
-from unittest import TestCase
-
 from ..nbbase import (
     NotebookNode,
     new_code_cell, new_text_cell, new_worksheet, new_notebook
@@ -20,7 +18,8 @@ from .nbexamples import nb0, nb0_py
 def open_utf8(fname, mode):
     return io.open(fname, mode=mode, encoding='utf-8')
 
-class NBFormatTestCase(TestCase):
+class NBFormatTest:
+    """Mixin for writing notebook format tests"""
 
     # override with appropriate values in subclasses
     nb0_ref = None
@@ -61,4 +60,4 @@ class NBFormatTestCase(TestCase):
             nb = self.mod.read(f)
         
 
-        
+
diff --git a/IPython/nbformat/v3/tests/test_json.py b/IPython/nbformat/v3/tests/test_json.py
index 5b7d6f8..bd5f8a5 100644
--- a/IPython/nbformat/v3/tests/test_json.py
+++ b/IPython/nbformat/v3/tests/test_json.py
@@ -10,7 +10,7 @@ from . import formattest
 from .nbexamples import nb0
 
 
-class TestJSON(formattest.NBFormatTestCase):
+class TestJSON(formattest.NBFormatTest, TestCase):
 
     nb0_ref = None
     ext = 'ipynb'
diff --git a/IPython/nbformat/v3/tests/test_nbpy.py b/IPython/nbformat/v3/tests/test_nbpy.py
index 1b70d1d..858a6b7 100644
--- a/IPython/nbformat/v3/tests/test_nbpy.py
+++ b/IPython/nbformat/v3/tests/test_nbpy.py
@@ -1,11 +1,14 @@
 # -*- coding: utf8 -*-
+
+from unittest import TestCase
+
 from . import formattest
 
 from .. import nbpy
 from .nbexamples import nb0, nb0_py
 
 
-class TestPy(formattest.NBFormatTestCase):
+class TestPy(formattest.NBFormatTest, TestCase):
 
     nb0_ref = nb0_py
     ext = 'py'