[py3] Fixed a loop that changed dictionary size.

django/db/models/fields/__init__.py

@@ -489,7 +489,7 @@ def formfield(self, form_class=forms.CharField, **kwargs):
             # Many of the subclass-specific formfield arguments (min_value,
             # max_value) don't apply for choice fields, so be sure to only pass
             # the values that TypedChoiceField will understand.
-            for k in kwargs.keys():
+            for k in list(six.iterkeys(kwargs)):
                 if k not in ('coerce', 'empty_value', 'choices', 'required',
                              'widget', 'label', 'initial', 'help_text',
                              'error_messages', 'show_hidden_initial'):