Update: After realizing that PyYAML 5.1 had introduced the use of the python_requires argument, I had to rephrase this issue completely.

The python_requires argument of setup() specified since PyYAML 5.1 is not honored by pip. For example, pip install pyyaml on Python 3.4 happily installs PyYAML 5.1.2 which does not support Python 3.4.

That is consistent with the fact that the Pypi page for PyYAML does not show the required Python versions in its Meta section. Compare that for example with the Python versions shown in the Meta section of the Pypi page for pywbem.

I don't have immediate advice how to fix that, but I think it has to do with how the package is built or uploaded to Pypi.