[wtforms] fix SelectField.choices

`list` is invariant, so .choices can't be assigned to after initialisation. Doing so is suggested in the documentation:

https://wtforms.readthedocs.io/en/3.2.x/fields/#wtforms.fields.SelectField

Author: marksteward

stubs/WTForms/wtforms/fields/choices.pyi

@@ -46,7 +46,7 @@ class SelectFieldBase(Field):
 
 class SelectField(SelectFieldBase):
     coerce: Callable[[Any], Any]
-    choices: list[_Choice] | _GroupedChoices
+    choices: Sequence[_Choice] | _GroupedChoices
     validate_choice: bool
     def __init__(
         self,