dev-python/matplotlib: Replace python_gen_usedep with python_gen_cond_dep

An upgrade attempt was made in CL:2328827, but it fails because of
reverse depedencies that want python 2* support.

BUG=chromium:1111336
TEST=emerge matplotlib

Change-Id: Ie416b97a7fe60d5a576722e67afca1b16517eb7d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/portage-stable/+/2333121
Commit-Queue: Allen Webb <allenwebb@google.com>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Allen Webb <allenwebb@google.com>
Auto-Submit: Allen Webb <allenwebb@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
2 files changed