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.
* python/types.c python/tests/Makefile.am python/tests/xpathret.py:
added the possibility of returning nodesets from XPath extension
functions written in Python
Daniel