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.
* tree.c python/tests/Makefile.am python/tests/attribs.py:
fixed xmlHasNsProp() bugs for defaulted from DTD attribs,
added a specific regression test
* python/generator.py: xmlHasNsProp() and xmlHasProp() shall
not raise exceptions when failing to find the attribute.
Daniel