NBFormatTest is now a mixin, rather than a base class

IPython/nbformat/v3/tests/formattest.py

@@ -6,8 +6,6 b' 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 b' 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

IPython/nbformat/v3/tests/test_json.py

@@ -10,7 +10,7 b' from . import formattest'
 from .nbexamples import nb0
 
 
-class TestJSON(formattest.NBFormatTestCase):
+class TestJSON(formattest.NBFormatTest, TestCase):
 
     nb0_ref = None
     ext = 'ipynb'

IPython/nbformat/v3/tests/test_nbpy.py

@@ -1,11 +1,14 b''
 # -*- 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'