Python 2 & 3 compatibility: part 2

This is an attempt to solve #56 (closed) by breaking up !545 (closed) and adjusting changes. This MR includes commits 6-11:

• 8ff19358
• b18ec813
• a9be3d4d
• 4ea33ad7
• aaf2817b: superseded as ${boost_python_version} and $ENV{HEPTOOLS_VERSION} have been added
• 69395ae3

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 !832 (merged) and I will rebase once that has happened (just want to run the tests right now).