some more minor fixes to the buffer docs

Python docs says they should not be used in extension modules.
https://docs.python.org/3/c-api/gcsupport.html#c._PyObject_GC_TRACK

do not use cythonize() but Cython.Distutils in setup.py to avoid Cython compilation when not building the extensions

Conflicts:
	setup.py

clean up extension building in setup.py: clarify what's Py3.2-specific and use cythonize() instead of Cython.Distutils

undo incorrect coercion removal - passing through C types might be required due to low-level value conversions