It is possible to make xmlIO handle any protocol by means of
xmlRegisterInputCallback(). However, that function is currently only
available in C API. So, the natural solution seems to be implementing Python
bindings for the xmlRegisterInputCallback.
* python/generator.py: skip xmlPopInputCallbacks
* python/libxml.c python/libxml.py python/libxml_wrap.h: implement the
wrappers
* python/tests/input_callback.py python/tests/Makefile.am: also add a test case
