Updates for python3 "native"

Make changes to support distributions where Python is provided as python3 (such as Ubuntu 22.04).

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