Following the addition of pyproject.toml, we should specify minimal supported version there, so wheels are always built with the oldest supported numpy (for each supported Python version). Because we use the scipy cython BLAS API wheels should also be built with oldest supported scipy (also Python version dependent).

Specifying "numpy>=1.13.3", "scipy>=0.19.1" is not enough I think.

See how it is done in,

• scipy: https://github.com/scipy/scipy/blob/master/pyproject.toml
• pandas: https://github.com/pandas-dev/pandas/blob/master/pyproject.toml
• or https://pypi.org/project/oldest-supported-numpy/

Should be an easy fix, but marking as a blocker for the release. Somewhat related #15917

Discovered while working on scikit-learn-contrib/scikit-learn-extra#51