Replace `ModelMultipleChoiceField`'s `list` error message with `invalid_list`

[Thibaut Decombe]

Description

Since django 3.1, the forms.ModelMultipleChoiceField list error message key is deprecated in favor of invalid_list.

SImilarly to the EmailValidator(whitelist=...), I think this could be auto-fixed.

-forms.ModelMultipleChoiceField(error_messages={"list": "Enter multiple values."})
+forms.ModelMultipleChoiceField(error_messages={"invalid_list": "Enter multiple values."})

Would you be interested in a PR ?

[Adam Johnson]

Yeah sure!