Skip validate only on constant readonly fields

A field can be readonly by the domain inversion in the validate method. So it
should be validated again in case its domain inversion change and reset it a
non-readonly.
So only field that are always readonly can be skipped for the validation.

issue6448
review37351002

tryton/gui/window/view_form/model/record.py

@@ -401,7 +401,7 @@ class Record(SignalEvent):
         for field_name, field in self.group.fields.iteritems():
             if fields is not None and field_name not in fields:
                 continue
-            if field.get_state_attrs(self).get('readonly', False):
+            if field.attrs.get('readonly'):
                 continue
             if field_name == self.group.exclude_field:
                 continue