Fix listify in 04_callbacks notebook (#369)
Fix listify in 04_callbacks notebook (#369)

Fixes #368
Tests in 08_data_block notebook did not pass because of faulty listify implementation.
fastai/course-v3,fastai/course-v3,fastai/course-v3,fastai/course-v3,fastai/course-v3,fastai/course-v3,fastai/course-v3