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.
* relaxng.c: some cleanups
* doc/xmlreader.html: extended the document to cover RelaxNG and
tree operations
* python/tests/Makefile.am python/tests/reader[46].py: added some
xmlReader example/regression tests
* result/relaxng/tutor*.err: updated the output of a number of tests
Daniel