Tests break with pytest 7.2.0

[FriedrichFroebel]

CI builds started breaking today due to pytest 7.2.0 removing the py.test import support. At the time of writing this, the changelog on the website does not yet include this new release, but GitHub already provides the corresponding data. The offending change is pytest-dev/pytest#10396.

Errors appear like this:

______________________ ERROR collecting tests/test_xml.py ______________________
ImportError while importing test module '/home/runner/work/gamera-4/gamera-4/tests/test_xml.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/opt/hostedtoolcache/Python/3.8.14/x64/lib/python3.8/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/test_xml.py:1: in <module>
    import py.test
E   ModuleNotFoundError: No module named 'py.test'; 'py' is not a package

The solution is to simply replace the imports and usages of py.test with pytest. I will open a corresponding PR when I find some more time in the next days.