Patchwork doesn't run on Xenial with system packages

[Daniel Axtens]

Hi,

If I try to run patchwork with system versions of packages on Xenial, I
run in to problems.

Specifically, when I run with ENABLE_REST_API=True, I see the following
error:

ImportError: Could not import
'django_filters.rest_framework.DjangoFilterBackend' for API setting
'DEFAULT_FILTER_BACKENDS'. ImportError: No module named rest_framework.

I suspect this is due to the version of DRF - apt search shows:

  python-djangorestframework/xenial 3.3.2-2 all

requirements-prod.txt lists the required version as 3.5 to 3.6.

This is something we really need to fix: ozlabs.org runs Debian, and if
it doesn't run on Xenial it's not likely to run on Debian. And we
really, really want ozlabs.org to be able to support the REST API.

Can we relax the drf requirement?

Regards,
Daniel