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/persistent/tests/test_overriding_attrs.py

@@ -377,5 +377,9 @@ class SampleOverridingGetattributeSetattrAndDelattr(Persistent):
 
 
 def test_suite():
-    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
     return DocTestSuite()

src/persistent/tests/test_persistent.py

@@ -12,7 +12,12 @@
 #
 ##############################################################################
 from persistent import Persistent, simple_new
-import doctest
+import os
+if os.environ.get('USE_ZOPE_TESTING_DOCTEST'):
+    from zope.testing import doctest
+else:
+    import doctest
+
 import unittest
 
 class P(Persistent):

src/persistent/tests/test_pickle.py

@@ -264,7 +264,12 @@ def test_pickling_w_slots_w_empty_dict():
 
     """
 
-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
+
 import unittest
 
 def test_suite():

src/persistent/tests/test_wref.py

@@ -12,7 +12,11 @@
 #
 ##############################################################################
 import unittest
-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
 
 def test_suite():
     return DocTestSuite('persistent.wref')