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.
* xmlreader.c include/libxml/xmlreader.h: Added the Expand()
and Next() operation to work on subtrees within the reader
framework.
* doc/libxml2-api.xml python/libxml2class.txt: resulting updates
* python/tests/reader5.py: added an example for those new
functions of the reader.
Daniel