python-django: security bump to version 1.7.3

Fixes:

CVE-2015-0219 - incorrectly handled underscores in WSGI headers. A
remote attacker could possibly use this issue to spoof headers in
certain environments.

CVE-2015-0220 - incorrectly handled user-supplied redirect URLs. A
remote attacker could possibly use this issue to perform a cross-site
scripting attack.

CVE-2015-0221 - incorrectly handled reading files in
django.views.static.serve(). A remote attacker could possibly use this
issue to cause Django to consume resources, resulting in a denial of
service.

CVE-2015-0222 - incorrectly handled forms with ModelMultipleChoiceField.
A remote attacker could possibly use this issue to cause a large number
of SQL queries, resulting in a database denial of service.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>