As per https://peps.python.org/pep-0394/, the python binary can be one
of the following options:
- Python 2
- Python 3
- Not exist
All of the scripts in libxml2 use 'python', which may not exist.
As Python 2 reached EOL on the 1st January 2020, it's safe to move the
scripts to use python3 explicitly.
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