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.
* parserInternals.c xmlIO.c encoding.c include/libxml/parser.h
include/libxml/xmlIO.h: added xmlByteConsumed() interface
* doc/*: updated the benchmark rebuilt the docs
* python/tests/Makefile.am python/tests/indexes.py: added a
specific regression test for xmlByteConsumed()
* include/libxml/encoding.h rngparser.c tree.c: small cleanups
Daniel