The problem was, there was some field labels that had non ascii symbols. When I changed to it ascii only symbols, then this warning disappeared.
For example let say it was something like this:
_columns = {
'some_field': fields.char('Field label with non ascii ęą'),
}
When I changed to:
_columns = {
'some_field': fields.char('Field label with ascii only'),
}
Warning disappeared.