Merge r121830 from /ZODB/branches/jim-python-btrees.

Tests still don't pass. :(

BTrees/tests/testBTrees.py

@@ -1903,6 +1903,12 @@ class BTreeTests(MappingBase):
                 self.assertEqual(str(detail), "the bucket being iterated "
                                               "changed size")
                 break
+            except KeyError, v:
+                # The Python implementation behaves very differently and
+                # gives a key error in this situation. It can't mess up
+                # memory and can't readily detect changes to underlying buckets
+                # in any sane way.
+                self.assertEqual(str(v), str(k[0]))
         self._checkIt(t)
 
 class IIBTreeTest(BTreeTests, unittest.TestCase):