The stdlib doctest.DocTestSuite is broken in such a way that you can't

repeat tests.  Provide an environment variable to use the doctest from
zope.testing.

src/ZODB/tests/testBroken.py

@@ -18,7 +18,11 @@ import sys
 import unittest
 import persistent
 import transaction
-from doctest import DocTestSuite
+import os
+if os.environ.get('USE_ZOPE_TESTING_DOCTEST'):
+    from zope.testing.doctest import DocTestSuite
+else:
+    from doctest import DocTestSuite
 from ZODB.tests.util import DB
 
 def test_integration():

src/ZODB/tests/testblob.py

@@ -20,7 +20,12 @@ from ZODB.DB import DB
 from ZODB.FileStorage import FileStorage
 from ZODB.tests.testConfig import ConfigTestBase
 
-import doctest
+import os
+if os.environ.get('USE_ZOPE_TESTING_DOCTEST'):
+    from zope.testing import doctest
+else:
+    import doctest
+
 import os
 import random
 import re