updating build dependencies for zope3 packages (dh_pycentral -> dh_python2)

[Matthias Klose]

this only affects zope3, please ignore for zope2* packaging.

I'd like to get rid off the python-central (build-)dependencies, and would like 
to replace it by dh_python2.  The changes look mechanical, but I'd like to see 
van.pydeb converted first, and then start to convert the other zope3 packages. 
Any objections on this proposal?

   Matthias