SelectField & SelectMultipleField dynamic choices raising error on WTForms >= 2.3.0

[sethkaufee]

First off, thank you for publishing this package. Appreciate the responses and issue closing recently.

With the recent WTForms update, any lambda or other values that cannot be coerced to a list will fail on initialization since the new SelectField now copies the value with list(choices). Before, the values were being copied using the copy library, copy(choices). To support callables (https://wtforms-components.readthedocs.io/en/latest/#selectfield-selectmultiplefield) in the new version, it looks like the __init__ might have to be overridden to continue to support dynamic choices.

I'm not sure how much the will hurt backwards compatibility but version 2.2.1 looks like:

    def __init__(self, label=None, validators=None, coerce=text_type, choices=None, **kwargs):
        super(SelectField, self).__init__(label, validators, **kwargs)
        self.coerce = coerce
        self.choices = copy(choices)

and the new looks like:

    def __init__(
        self,
        label=None,
        validators=None,
        coerce=str,
        choices=None,
        validate_choice=True,
        **kwargs,
    ):
        super().__init__(label, validators, **kwargs)
        self.coerce = coerce
        self.choices = list(choices) if choices is not None else None
        self.validate_choice = validate_choice

Deferring the choices initialization might cover most bases (although haven't ran the code):

class SelectField(_SelectField):
    def __init__(self, *args, choices=None, **kwargs):
        if callable(choices):
            super().__init__(*args, **kwargs)
            self.choices = copy(choices)
        else:
            super().__init__(*args, choices=choices, **kwargs)

I would be happy to open a PR on this or work on another solution if you have one. Thanks again!