Merged requested to merge rburton/libxml2:py into master
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.