use oldest-supported-numpy as build requirements

[xmatthias]

use oldest-supported-numpy for build requirements.
Explanation: pypa/pip#9542 (comment)

Otherwise due to a pip-bug, at build time, the latest numpy is installed, which can break if the environment uses a older pinned version.

in pypa/pip#9542, I've posted a snippet to recreate the faulty behavior with the current version.
As proposed there, using oldest-supported-numpy should fix this, as older binaries work with newer versions:

The reason to use the oldest available Numpy version as a build-time dependency is because of ABI compatibility. Binaries compiled with old Numpy versions are binary compatible with newer Numpy versions, but not vice versa.

Source

[roebel]

Thanks for the suggestion.