dbmigrate: fix python3 metaclasses error

rhodecode/lib/dbmigrate/migrate/changeset/schema.py

@@ -166,7 +166,39 @@ def _to_index(index, table=None, engine=None):
     return ret
 
 
-class ColumnDelta(DictMixin, sqlalchemy.schema.SchemaItem):
+
+# Python3: if we just use:
+#
+#     class ColumnDelta(DictMixin, sqlalchemy.schema.SchemaItem):
+#         ...
+#
+# We get the following error:
+# TypeError: metaclass conflict: the metaclass of a derived class must be a
+# (non-strict) subclass of the metaclasses of all its bases.
+#
+# The complete inheritance/metaclass relationship list of ColumnDelta can be
+# summarized by this following dot file:
+#
+# digraph test123 {
+#     ColumnDelta -> MutableMapping;
+#     MutableMapping -> Mapping;
+#     Mapping -> {Sized Iterable Container};
+#     {Sized Iterable Container} -> ABCMeta[style=dashed];
+#
+#     ColumnDelta -> SchemaItem;
+#     SchemaItem -> {SchemaEventTarget Visitable};
+#     SchemaEventTarget -> object;
+#     Visitable -> {VisitableType object} [style=dashed];
+#     VisitableType -> type;
+# }
+#
+# We need to use a metaclass that inherits from all the metaclasses of
+# DictMixin and sqlalchemy.schema.SchemaItem. Let's call it "MyMeta".
+class MyMeta(sqlalchemy.sql.visitors.VisitableType, abc.ABCMeta, object):
+    pass
+
+
+class ColumnDelta(six.with_metaclass(MyMeta, DictMixin, sqlalchemy.schema.SchemaItem)):
     """Extracts the differences between two columns/column-parameters
 
         May receive parameters arranged in several different ways: