Changing _check_value from:

    def _check_value(self, action, value):
        # converted value must be one of the choices (if specified)
        if action.choices is not None and value not in action.choices:
            ...

to

    def _check_value(self, action, value):
        # converted value must be one of the choices (if specified)
        if action.choices is not None:
            choices = action.choices
            if isinstance(choices, str):
                choices = list(choices)
            if value not in action.choices:
                ...

would correct the string search without affecting other types of choices.