Change explicit mention of Python binary to python3
which should be supported by all distributions and
environments (https://peps.python.org/pep-0394/).

Update hash bangs to python3.

Adapt CMake to search for pytest also as pytest-3; likewise
for nosetest binary as nosetests3 (with fallback to
pytest and nosetests, which can be used in some
virtual environments, even with a Python 3 install).

Remove unnecessary utf-8 encoding lines.

Update test scripts to python3