Added widget_type annotation to BoundField (#855)

* Add widget_type annotation to BoundField

See https://github.com/django/django/blob/main/django/forms/boundfield.py#L283

* Tweaked annotation to be `AnyStr`, not `str`

* Reverted to use `str` as return type

django-stubs/forms/boundfield.pyi

@@ -47,6 +47,8 @@ class BoundField:
     def build_widget_attrs(
         self, attrs: Dict[str, str], widget: Optional[Widget] = ...
     ) -> Dict[str, Union[bool, str]]: ...
+    @property
+    def widget_type(self) -> str: ...
 
 class BoundWidget:
     parent_widget: Widget = ...