Do not send value for readonly Many2Many

In contrary to One2Many, Many2Many can not be modified when they are readonly.
But Many2Many inherit from One2Many so we must exclude them explicitly from
the test.

issue8323
review275371002

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

@@ -254,7 +254,8 @@ class Record(SignalEvent):
         value = {}
         for name, field in self.group.fields.items():
             if (field.attrs.get('readonly')
-                    and not isinstance(field, fields.O2MField)):
+                    and not (isinstance(field, fields.O2MField)
+                        and not isinstance(field, fields.M2MField))):
                 continue
             if field.name not in self.modified_fields and self.id >= 0:
                 continue