Add Python 3 to CI
- Add a build and a test job using LCG 96python3
- Build on top of !545 (closed) by @apearce, rebasing and adapting where needed
- All tests passing with both python2 and python3!
- Supersedes !545 (closed) and !864 (closed)
Closes #56 (closed)
Edited by Marco Clemencic