The PyBytes_InternInPlace 2-to-3 transition macro is missing in 
include/bytesobject.h

Alternatively, the documentation[1] is missing some details. At the top 
there's a note saying "These functions have been renamed to PyBytes_* in 
Python 3.x. The PyBytes names are also available in 2.6.".

[1] http://docs.python.org/c-api/string.html

Confirmed with version 2.6.1 (original release from python.org)