Boost.Python should use macros to access members of PyMethodObject

[Amaury Forgeot d'Arc <amauryfa@…>]

In libs/python/src/wrapper.cpp, the members of PyMethodObject are accessed directly:

  ((PyMethodObject*)m.get())->im_self
  ((PyMethodObject*)m.get())->im_func

These accesses are not documented in the python API. I suggest to use the following macros instead, which have been existing since python 1.6 at least:

  PyMethod_GET_SELF(m.get())
  PyMethod_GET_FUNCTION(m.get())

When using CPython, these macros expand to exactly the same code.

The goal is to allow other implementations of python, like PyPy, to provide alternate compatible implementation of these macros, and thus be able to load modules compiled with Boost.Python.