fix: add o2o field does not create constraint when migrating (#396)

* fix: add o2o field does not create constraint when migrating

* Add testcase and update changelog

* docs: update migrating list

* refactor: use `_handle_o2o_fields` instead of `is_o2o=True`

* Remove unused line

tests/models.py

@@ -40,6 +40,7 @@ class Email(Model):
     is_primary = fields.BooleanField(default=False)
     address = fields.CharField(max_length=200)
     users: fields.ManyToManyRelation[User] = fields.ManyToManyField("models.User")
+    config: fields.OneToOneRelation["Config"] = fields.OneToOneField("models.Config")
 
 
 def default_name():
@@ -91,6 +92,8 @@ class Config(Model):
         "models.User", description="User"
     )
 
+    email: fields.OneToOneRelation["Email"]
+
 
 class NewModel(Model):
     name = fields.CharField(max_length=50)