Use python3 not python

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/generator.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # generate python wrappers from the XML API description
 #