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.
* build_glob.py global.data globals.c parser.c
include/libxml/globals.h: patch from Stphane Bidoul for setting
up threads global defaults.
* doc/libxml2-api.xml: this extends the API with new functions
* python/tests/Makefile.am python/tests/reader2.py
python/tests/thread2.py: integrated the associated testcase and
fixed the error string used in reader2
Daniel