Remove python type inherited from Many2Many of One2One

The python type is used to test the type of value set but for One2One it is an
instance which can not be tested with a simple Python type.

Closes #13817

trytond/trytond/model/fields/one2one.py

@@ -8,6 +8,7 @@ from trytond.pool import Pool
 
 class One2One(Many2Many):
     _type = 'one2one'
+    _py_type = None
 
     def get(self, ids, model, name, values=None):
         '''