fix bug when removing attributes from source outside the permitted attributes

src/ldaptools/synchronize.py

@@ -228,7 +228,7 @@ class Synchronize(object):
         entries = list(self.source)
         entries.sort(key=lambda (dn, entry): len(str2dn(dn)))
         for dn, entry in entries:
-            for key in entry:
+            for key in entry.keys():
                 if not key in self.attributes:
                     del entry[key]
         # First create, rename and update