Hedge when using the transaction iterator's close method.

The close() method is not in the interface and is not provided
by ZEO's implementation.

src/ZODB/tests/IteratorStorage.py

@@ -146,7 +146,8 @@ class IteratorStorage(IteratorCompare):
         tinfo = next(it)
         self.assertEqual(1, len(list(tinfo)))
         self.assertEqual(1, len(list(tinfo)))
-        it.close()
+        if hasattr(it, 'close'):
+            it.close()
 
     def checkIterateWhileWriting(self):
         self._dostore()

src/ZODB/tests/hexstorage.py

@@ -87,7 +87,8 @@ class HexStorage(object):
             for t in it:
                 yield Transaction(self, t)
         finally:
-            it.close()
+            if hasattr(it, 'close'):
+                it.close()
 
     def storeBlob(self, oid, oldserial, data, blobfilename, version,
                   transaction):