This is a meta-package which can be used in pyproject.toml files to automatically provide the oldest supported version of Numpy without having to list them all. In other words:

[build-system]
requires = [
    "wheel",
    "setuptools",
    "numpy==1.13.3; python_version=='3.5',
    "numpy==1.13.3; python_version=='3.6',
    "numpy==1.14.5; python_version=='3.7',
    "numpy==1.17.3; python_version>='3.8'
]

can be replaced by:

[build-system]
requires = ["wheel", "setuptools", "oldest-supported-numpy"]

And as new Python versions are released, the pyproject.toml file does not need to be updated.