[FIX] core: make accessing fields from __str__ possible

Because CacheMiss calls str() on the record it's given, it's not
possible to override __str__ and get field values: on the first cache
miss (= the first field access) we're going to blow the stack:

* any random field is accessed
* and faults
* which creates a CacheMiss
* which stringifies the record
* which accesses a field
* which faults
* which creates a CacheMiss
* etc...

Fix:

* move the basic Model's stringification to repr(), which is
  implicitly called on str() if no __str__ is specified, for historical
  reasons we were doing the opposite
* use repr() when initialising CacheMiss

closes odoo/odoo#42966

Signed-off-by: Xavier Morel (xmo) <xmo@odoo.com>

odoo/exceptions.py

@@ -104,7 +104,7 @@ class CacheMiss(except_orm, KeyError):
     """
 
     def __init__(self, record, field):
-        super(CacheMiss, self).__init__("%s.%s" % (str(record), field.name))
+        super(CacheMiss, self).__init__("%r.%s" % (record, field.name))
 
 
 class MissingError(except_orm):

odoo/models.py

@@ -5665,10 +5665,8 @@ Record ids: %(records)s
     def __int__(self):
         return self.id or 0
 
-    def __str__(self):
-        return "%s%s" % (self._name, getattr(self, '_ids', ""))
     def __repr__(self):
-        return str(self)
+        return "%s%s" % (self._name, getattr(self, '_ids', ""))
 
     def __hash__(self):
         if hasattr(self, '_ids'):