Description
The QML library cannot be installed in an environment where NumPy is not already installed, for example a fresh virtual environment when installing another project which uses QML as a dependency.
To reproduce
python3 -m venv venv
source venv/bin/activate
pip install qml

If NumPy is installed beforehand, then it works
python3 -m venv venv
source venv/bin/activate
pip install numpy
pip install qml

How to fix
NumPy should be specified as a dependency in the setup configuration.