patch to pass Python pytest checks (applies only to Linux)

[nbehrnd]

Changes on side of the PubChem database and Python's syntax broke some of the checks with pytest defined until April 2017. Except for probing the readout of mmff94_energy_3d now commented out, each of them pass again in GitHub's runner image of Ubuntu 24.04 with Python 3.13.2 and pytest 8.3.5. The additional pyproject.toml file includes a constraining classifier

"Operating System :: POSIX :: Linux",

as a brief reminder about the difficulties to run pubchempy.py in Windows.