Prepare deb packaging for Py2+3 on master

 - [ ] adapt build scripts to include obspy in both the stable Python 2.x *and 3.x* environments (actually this will have to be two different packages..)
 - [x] ~~resolve `suds-jurko` (no deb packages so far; optionally remove all `suds` usage)~~ see #806
 - [ ] resolve `future` (no deb packages so far)