Building for Python3.6 - ImportError: undefined symbol: PyClass_Type

Hi,

I am trying to build boost for use with Python 3 but I am having some troubles.
I call `./bootstrap --with-python-version=3.6` and then `./b2 --with-python --debug-configuration` which correctly shows python 3.6 is being used and the python component is being built.

When running the `$BOOST_ROOT/libs/pythion/example/tutorial`, I get the following error:
```ImportError: /usr/lib/x86_64-linux-gnu/libboost_python-py27.so.1.65.1: undefined symbol: PyClass_Type```

This shows the python2.7 shared object is being used and I am aware that the PyClass_Type is a Python 2 type. So somehow the program is still being built with python 2 despite me specifying Python 3.
I have tried several iterations of cleaning and building all, just in case something had not been built properly, but I still get the same issue.

Am I missing something ?

Thanks