Fix SlugRelatedField.slug_field type: cannot be None (#830)

Author: lev-blit

rest_framework-stubs/relations.pyi

@@ -135,7 +135,7 @@ class HyperlinkedRelatedField(RelatedField[_MT, str, Hyperlink]):
 class HyperlinkedIdentityField(HyperlinkedRelatedField): ...
 
 class SlugRelatedField(RelatedField[_MT, str, str]):
-    slug_field: str | None
+    slug_field: str
     def __init__(
         self,
         slug_field: str,