Veusz - update to 3.0

[Jeremy Sanders <jeremy@jeremysanders.net>]

Dear all

I'd be grateful if someone could have a look at the new packaging for 
Veusz 3.0, which now includes 3D plotting.

Please see https://salsa.debian.org/python-team/applications/veusz

The main issue is probably whether the new split into multiple 
subpackages is ok or too much. It's set up to have

veusz  (main executable, examples and docs, arch all)
python3-veusz (python3 module, arch all)
python3-veusz.helpers (python3 arch-dependent bits, arch any)
python-veusz (python2 module, arch all)
python-veusz.helpers (python2 arch-dependent bits, arch any)
veusz-data (shared data files by pythonX-veusz, arch any)

veusz depends on python3-veusz
python3-veusz depends on python3-veusz.helpers and veusz-data
python-veusz depends on python-veusz.helpers and veusz-data

There are also debug packages for the python2 and 3 helpers packages.

I could simplify things by dropping python2, or folding the arch 
dependent and independent parts together.

Another issue is that I can't get continuous integration tests working 
properly on Salsa. The module works fine in a pbuilder build, but on 
Salsa, Qt can't load its xcb module for the self tests under Xvfb.

Thanks

Jeremy Sanders