setuptools is a run-time dependency (sic!), all others are required
for testing only.
* gnu/packages/django.scm (python-django) [propagated-inputs,
native-inputs]: Exchange them. Move tzdata to new native-inputs.
Enhance comments.
(python2-django) [propagated-inputs] change into native-inputs.
Enhance comment.
Signed-off-by: Ludovic Courtès <ludo@gnu.org>