5.0: Update django.db.models.constraints.UniqueConstraint (#2175)

Reflects the nulls_distinct option of the UniqueConstraint class. Adds
the attribute and the arg to the two versions of the __init__ type
definitions.

https://docs.djangoproject.com/en/5.0/ref/models/constraints/#nulls-distinct

django-stubs/db/models/constraints.pyi

@@ -53,6 +53,7 @@ class UniqueConstraint(BaseConstraint):
     fields: Sequence[str]
     condition: Q | None
     deferrable: Deferrable | None
+    nulls_distinct: bool | None
 
     @overload
     def __init__(
@@ -64,6 +65,7 @@ class UniqueConstraint(BaseConstraint):
         deferrable: Deferrable | None = ...,
         include: Sequence[str] | None = ...,
         opclasses: Sequence[Any] = ...,
+        nulls_distinct: bool | None = ...,
         violation_error_code: str | None = ...,
         violation_error_message: _StrOrPromise | None = ...,
     ) -> None: ...