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.
* Makefile.am doc/examples/Makefile.am python/tests/Makefile.am
xstc/Makefile.am: try to fix a problem with valgrind.
* python/generator.py python/libxml.c python/tests/Makefile.am
python/tests/tstmem.py: applied memory leak fix from Brent Hendricks
c.f. bug #165349
Daniel