use oldest-supported-numpy as build requirements

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
Explanation: pypa/pip#9542 (comment)

Author: xmatthias

pyproject.toml

@@ -1,3 +1,3 @@
 [build-system]
 # Minimum requirements for the build system to execute.
-requires = ["setuptools", "numpy>=1.13.0", "wheel"]  # PEP 508 specifications.
+requires = ["setuptools", "oldest-supported-numpy", "wheel"]  # PEP 508 specifications.