fix broken tests on django 5.1.1 / python 3.12

failing with exception in
`django.db.models.fields.related_descriptors.ReverseOnToOneDescriptor.__get__`
due to missing `accessor_name` attr/property, which appears to be a
result of
django/django@5e80390

relativity/fields.py

@@ -363,6 +363,10 @@ def field(self):
     def get_accessor_name(self):
         return self.name
 
+    @cached_property
+    def accessor_name(self):
+        return self.get_accessor_name()
+
     def _get_extra_restriction(self, related_alias, local_alias):
         return Restriction(
             forward=True,