Missing _py_type in one2one model fields
1. Try run a module test in version 7.4:
```
======================================================================
ERROR: test (test_scenario_abort_event_test.Test.test)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/tryton/trytond/trytond/modules/farm/tests/test_scenario_abort_event_test.py", line 113, in test
AbortEvent.validate_event([abort_female.id], config.context)
File "/tryton/tryton/proteus/proteus/config.py", line 211, in __call__
result = rpc.result(meth(*c_args, **c_kwargs))
^^^^^^^^^^^^^^^^^^^^^^^^^
File "/tryton/tryton/trytond/trytond/model/modelview.py", line 775, in wrapper
return func(cls, records, *args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/tryton/tryton/trytond/trytond/model/workflow.py", line 37, in wrapper
result = func(cls, filtered, *args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/tryton/tryton/trytond/trytond/modules/farm/events/abort_event.py", line 60, in validate_event
diagnosis_event.female_cycle = diagnosis_event.animal.current_cycle
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/tryton/tryton/trytond/trytond/model/fields/one2one.py", line 60, in __set__
Field.__set__(self, inst, value)
File "/tryton/tryton/trytond/trytond/model/fields/field.py", line 403, in __set__
value = self._py_type(value)
^^^^^^^^^^^^^^^^^^^^
TypeError: 'farm.animal.female_cycle' object is not iterable
```
2. Source code, save data in one2one field:
```
for diagnosis_event in events:
diagnosis_event.female_cycle = diagnosis_event.animal.current_cycle
diagnosis_event.save()
diagnosis_event.female_cycle.update_state(diagnosis_event)
```
3. Example debuging scenario data:
```
>>> diagnosis_event.animal.current_cycle.id
1
>>> diagnosis_event.id
1
>>> diagnosis_event.female_cycle
None
>>> diagnosis_event.female_cycle = diagnosis_event.animal.current_cycle
TypeError: 'farm.animal.group' object is not iterable
```
Since issue6460 add "Allow SQL expression as value of fields". I think in case one2one field, is missing _py_type attribute. In case apply next diff, the test return "OK" (succesfully).
```
diff --git a/trytond/trytond/model/fields/one2one.py b/trytond/trytond/model/fields/one2one.py
index 9d7207a62a..72ed23add6 100644
--- a/trytond/trytond/model/fields/one2one.py
+++ b/trytond/trytond/model/fields/one2one.py
@@ -8,6 +8,8 @@ from trytond.pool import Pool
class One2One(Many2Many):
_type = 'one2one'
+ _py_type = None
+ _sql_type = 'INTEGER'
def get(self, ids, model, name, values=None):
'''
```
issue