Python 3.13.0b1 support

* Prefer Cython3+ `old_build_ext`; required for 3.13+

pyproject.toml

@@ -1,3 +1,8 @@
 [build-system]
-requires = ["setuptools", "wheel", "Cython<3.0"]
+requires = [
+    "setuptools",  # FIXME: declare min/max setuptools versions?
+    "wheel",
+    "Cython; python_version < '3.13'",
+    "Cython>=3.0; python_version >= '3.13'"
+]
 build-backend = "setuptools.build_meta"

setup.py

@@ -34,6 +34,8 @@
     "Programming Language :: Python :: 3.9",
     "Programming Language :: Python :: 3.10",
     "Programming Language :: Python :: 3.11",
+    "Programming Language :: Python :: 3.12",
+    "Programming Language :: Python :: 3.13",
     "Programming Language :: Python :: Implementation :: CPython",
     "Programming Language :: Python :: Implementation :: PyPy",
     "Topic :: Software Development :: Libraries :: Python Modules",
@@ -82,7 +84,12 @@
     with_cython = True
 try:
     from Cython.Distutils.extension import Extension as _Extension
-    from Cython.Distutils import build_ext as _build_ext
+    try:
+        # try old_build_ext from Cython > 3 first, until we can dump it entirely
+        from Cython.Distutils.old_build_ext import old_build_ext as _build_ext
+    except ImportError:
+        # Cython < 3
+        from Cython.Distutils import build_ext as _build_ext
     with_cython = True
 except ImportError:
     if with_cython: