dev-python/django: Replace python_gen_usedep with python_gen_cond_dep

An upgrade attempt was made in CL:2328829, but it causes autotest
failures.

BUG=chromium:1111336
TEST=CQ passes

Change-Id: Idcad843c4e450e7a0c57e65c6e06a4e386514536
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/portage-stable/+/2333115
Tested-by: Allen Webb <allenwebb@google.com>
Auto-Submit: Allen Webb <allenwebb@google.com>
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
Commit-Queue: Allen Webb <allenwebb@google.com>
2 files changed