Fixed #13181 -- Added support for callable choices to forms.ChoiceField

Thanks vanschelven and expleo for the initial patch.

docs/releases/1.8.txt

@@ -240,6 +240,9 @@ Forms
   will also update ``UploadedFile.content_type`` with the image's content type
   as determined by Pillow.
 
+* You can now pass a callable that returns an iterable of choices when
+  instantiating a :class:`~django.forms.ChoiceField`.
+
 Generic Views
 ^^^^^^^^^^^^^