Fix dict being changed during iteration

.. for python3: simply using .keys() is not enough.

Author: schlatterbeck

roundup/hyperdb.py

@@ -954,8 +954,9 @@ def post_init(self):
         for cn in self.getclasses():
             cl = self.getclass(cn)
             # This will change properties if a back-multilink happens to
-            # have the same class, so we need to iterate over .keys()
-            for p in cl.properties.keys():
+            # have the same class, so we need to iterate over a list made
+            # from .keys()
+            for p in list(cl.properties.keys()):
                 prop = cl.properties[p]
                 if not isinstance (prop, (Link, Multilink)):
                     continue