Fix the type of form.Field.widget (#321)

The `widget` class attribute of `form.Field` expects a `Widget` class, not an instance of `Widget`.

See: https://docs.djangoproject.com/en/3.0/ref/forms/fields/#widget
Also usages at: https://github.com/django/django/blob/3259983f569151232d8e3b0c3d0de3a858c2b265/django/forms/fields.py#L46

django-stubs/forms/fields.pyi

@@ -14,7 +14,7 @@ class Field:
     initial: Any
     label: Optional[str]
     required: bool
-    widget: Widget = ...
+    widget: Type[Widget] = ...
     hidden_widget: Any = ...
     default_validators: Any = ...
     default_error_messages: Any = ...