Python3 compatibility

[smits]

urlgrabber is not available for python3 (at least not on OSX) so I replaced the urlgrabber usage in the python version of resource_retriever with the python built in urllib/urllib2.
In addition I replaced the home baked rospack_find by rospkg and added unit tests for the python api.

[smits]

Please take a look @jacquelinekay @mikepurvis

[mikepurvis]

Looks sane to me. Might make sense to add a travis config for ensuring coverage of py2 and py3, but that's up to the OSRF maintainers.

[smits]

@jacquelinekay Any chances on getting this in?

[mikepurvis]

I think it's @isucan maintaining this now.

This should definitely merge in preparation for the Lunar release.

[DLu]

I wrote the Python one, so I felt comfortable merging this in.

I know that @clalancette and @sloretz were taking over some of @jacquelinekay 's stuff (ros/urdf_parser_py#12). Perhaps this is another candidate.

[clalancette]

Yeah, we did take over some of @jacquelinekay 's stuff, but since this one seemed to be maintained by Ioan (Jackie was never listed in the package.xml), we haven't looked at it at all.

[wjwwood]

@clalancette if you look at the commits she was definitely maintaining this one too, Jackie probably just didn't update the package.xml. I'd consider this another package worth picking up. Though it doesn't necessarily mean you guys have to pick it up, it would make sense given the relationship between this package and the others.

It will be blocking the release of robot_model but also rviz, so I could probably pick it up too.

[clalancette]

Got it. Shane and I have now taken maintainership of it.