pyproject.toml: Prefer NumPy 2.0 in build-system.

Build against NumPy 2.0 in the build-system when possible (i.e., on Python 3.9 or later).
This will create code that is compatible with both NumPy 1.x and 2.x, see:

https://numpy.org/devdocs/dev/depending_on_numpy.html#numpy-2-0-specific-advice

pyproject.toml

@@ -9,6 +9,7 @@ requires = [
     "oldest-supported-numpy; python_version<'3.7'",
     # we need to build with at least 1.16.5 for a limited API bugfix
     "numpy==1.16.5; python_version=='3.7'",
-    "oldest-supported-numpy; python_version>='3.8'"
+    "oldest-supported-numpy; python_version=='3.8'",
+    "numpy>=2,<3; python_version>='3.9'",
 ]
 build-backend = "setuptools.build_meta"