change mapping of python-imaging since the package was removed from bionic

[dirk-thomas]

To address the failing CI from ros/ros_comm#1350 as well as the now failing binary job (http://build.ros.org/view/Msrc_uB/job/Mbin_uB64__rosbag__ubuntu_bionic_amd64__binary/3/).

[mikaelarguedas]

As python-pil is available on all Melodic targeted platforms, should we update the other ones ? maybe just artful as if doesn;'t share any platform with the other ROS distros ?

[dirk-thomas]

Please feel free to propose related changes. I just want this specific change to get merged since it blocks building melodic packages.

I don't see why any other mapping needs to change though.

[mikaelarguedas]

IDK it seems strange to target a different package for only one platform while the newly targeted package is available on all active Ubuntu and Debian distributions.
If you're confident that python-pil in bionic provides exactly the same behavior as python-imaging on other platforms and that using python-pil everywhere would break things I guess that's fine to merge as is

[tfoote]

Looking at python-imaging: https://packages.ubuntu.com/artful/python-imaging it appears to have just been a basic wrapper package that depends on python-pil for the full implementation.

python-pil has been available as far back as trusty: https://packages.ubuntu.com/artful/python-pil

We could consider switching all of the rules to python-pil but I think this is the more conservative approach without deeper research.