MNT: Build agains Numpy v2 where possible

Since it is not needed anymore with recent Numpy versions, and bump
minimum required Numpy to 2.0.0rc1 in order to be able to build Numpy
2-compatible wheels

pyproject.toml

@@ -3,5 +3,16 @@ requires = [
     "setuptools",
     "versioneer", 
     "wheel",
-    "oldest-supported-numpy"
+    "oldest-supported-numpy ; python_version < '3.9'",
+    # Comments on numpy build requirement range:
+    #
+    #   1. >=2.0.x is the numpy requirement for wheel builds for distribution
+    #      on PyPI - building against 2.x yields wheels that are also
+    #      ABI-compatible with numpy 1.x at runtime.
+    #   2. Note that building against numpy 1.x works fine too - users and
+    #      redistributors can do this by installing the numpy version they like
+    #      and disabling build isolation.
+    #   3. The <2.3 upper bound is for matching the numpy deprecation policy,
+    #      it should not be loosened
+    "numpy>=2.0.0rc1<2.3 ; python_version >= '3.9'",
 ]

setup.py

@@ -196,7 +196,10 @@ def get_long_description():
     install_requires=["numpy"],
     extras_require={"doc": ["numpydoc", "sphinx", "gitpython"]},
     cmdclass=cmdclass,
-    setup_requires=["numpy"],
+    setup_requires=[
+        "oldest-supported-numpy ; python_version < '3.9'",
+        "numpy>=2.0.0rc1 ; python_version >= '3.9'"
+    ],
     ext_modules=prepare_modules(),
     zip_safe=False,
 )