This only applies when using Python >=3.9 because older versions of Python do not suport Numpy 2.0 (see https://numpy.org/doc/stable/release/2.0.0-notes.html).
For versions of Python <3.9, the build system now uses the Python
package oldest-supported-numpy to install the earliest version of
Numpy supported by the version of Python used during compilation.
As a consequence, we could remove NUMPY_VERSION from the build system.