devel/boost-python-libs: unbreak in C++17 mode

In file included from libs/python/src/converter/from_python.cpp:6:
In file included from ./boost/python/converter/from_python.hpp:8:
In file included from ./boost/python/detail/prefix.hpp:13:
In file included from ./boost/python/detail/wrap_python.hpp:151:
In file included from /usr/local/include/python2.7/Python.h:88:
/usr/local/include/python2.7/unicodeobject.h:534:5: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]

register PyObject *obj,     /* Object */
^~~~~~~~~

/usr/local/include/python2.7/unicodeobject.h:553:5: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]

register PyObject *obj      /* Object */
^~~~~~~~~

/usr/local/include/python2.7/unicodeobject.h:575:5: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]

register const wchar_t *w,  /* wchar_t buffer */
^~~~~~~~~

/usr/local/include/python2.7/unicodeobject.h:593:5: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]

register wchar_t *w,        /* wchar_t buffer */
^~~~~~~~~