WIP: Python 2 & 3 compatibility: part 3

This is an attempt to solve #56 (closed) by breaking up !545 (closed) and adjusting changes. This MR includes commits 12-20:

• 2c6c49a7
• bdba77c7
• 8ea553d8
• 36251f3d
• f8e65227
• b096fbd9
• f1224765: superseded as ${boost_python_version} has been added
• 16c5f6c7
• 1717e8f3

Some rebasing, but not real changes were needed w.r.t Alex' commits. One has been dropped.

This MR is meant to be merged after !852 (merged) and I will rebase once that has happened (just want to run the tests right now).