Fix jam py3

[xnox]

Just build mpi-python against current boost_python, not boostpython2|3

Looks like boost_python has moved on and stopped providing
boost_python2 and boost_python3 libraries. Possibly a regression in
boost_python, or possibly new world order which mpi should adapt to.

Fixes build: --with-python --with-mpi --python-buildid=py36 python=3.6

[xnox]

I know that indenting is wrong, but the diff is easier to read/review this way.

[aminiussi]

So, unless I'm missing something, it seems we should go as described in boostorg/python#203.

Gonna give it a try then.

[Lastique]

This PR solves a different problem than what is described in boostorg/python#203, but it is a prerequisite. Without this change Boost.MPI fails to build because Boost.Python has changed the library target names. After this has been fixed, we can work on unifying library naming schemes, which is what boostorg/python#203 is about.

[aminiussi]

Any reason to merge on master instead of develop ?

[aminiussi]

It's merged on develop. Waiting for some tests to pass on develop and then merging on master.

[aminiussi]

Hi, @Lastique, it's in master, I did not merge your PR directly but made it pass through develop to check the test where ok on other platforms.

Thks

[Lastique]

Thanks. Although, it's not my PR.

[aminiussi]

Oups, sorry.

[xnox]

Thanks a lot! I have no idea about devel vs master branch differences, and I simply proposed something. Thank you for sorting out the branches merge targets et. al.