gaudi_add_pytest is used to declare pytest tests. Each of the test
files discovered by pytest are mapped to one ctest test, to be able to
run them in parallel.

Tests are collected when ctest is invoked and/or at build time, and only
in case changes are detected in scanned files.

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