I tried but failed to write a test to mimick numpy.int64 type. I tried to build a type which implements numbers.Rational but don't inherit from int, but there are way too many methods that I have to implement :-(

Morever, installing numpy on a Python 3.9 virtual environment is quite tricky. Lhe latest Cython release (0.29.14) isn't compatible with Python 3.9.

Miro gave me a command to install Cython on Python 3.9:

python -m pip install https://github.com/cython/cython/archive/master.tar.gz --install-option="--no-cython-compile"

But then "pip install numpy" tries to reinstall Cython which fails :-/