Discrepancy between python-tag definitions in Boost.Python and superproject

In Boost 1.67, Boost.Python uses its own definition of `python-tag` rule in `libs/python/Jamfile`. This rule produces library names like `libboost_python36-py36.so.1.67.0`, notice the 36 after `boost_python`. This is different from Boost 1.66 and from the similar rule defined in the superproject, in `boostcpp.jam`.

In particular, this results in different naming between Boost.Python and Boost.MPI Python binding libraries. I'm not sure why the duplicate Python version is needed, so this issue requests to port Boost.Python to the superproject definition of the rule.