[autoform] Make RelationField.fromcardinality a class method factory Instead of having the "RelationField" class hard-coded in this factory method (thus preventing any subclass to properly use it), we make that method a classmethod and instantiate the actual class instead of the base one.

[autoform] Use fieldclass from kwargs in guess_field for non-final relations This follows up on af969080e7e6 in which a "fieldclass" kwarg has been added in EntityFieldsForm.field_by_name() before calling guess_field(). In this changeset, this value was only used for final relation but the way non-final relation actually also changed since we also use guess_field from these relation (previously, the field class specified through uicfg was instantiated directly in EntityFieldsForm.field_by_name()). So in the non-final case in guess_field() we simply try to retrieve "fieldclass" from kwargs and fall back to RelationField.

[py37] `async` is now a reserved keyword