Add type annotation for Q.__xor__ (#240)

This PR adds a type annotation for `db.models.query_utils.Q.__xor__`.

* Documentation for `Q.__xor__` can be found [here](https://docs.djangoproject.com/en/dev/ref/models/querysets/#xor).
* An equivalent implementation in `django-stubs` can be found [here](https://github.com/typeddjango/django-stubs/blob/22706bf43cc7f0d4995670df8d72cff9d0bcf917/django-stubs/db/models/query_utils.pyi#L35).

Thanks in advance,
- Jakob

django-stubs/db/models/query_utils.pyi

@@ -37,6 +37,7 @@ class Q(tree.Node):
     def __init__(self, *args: Any, **kwargs: Any) -> None: ...
     def __or__(self, other: Any) -> Q: ...
     def __and__(self, other: Any) -> Q: ...
+    def __xor__(self, other: Any) -> Q: ...
     def __invert__(self) -> Q: ...
     def resolve_expression(
         self,