require Cython 3.0.3 at least, fixes #8133

The fix for the Python 3.12 memory leak issue was
in Cython 3.0.3+.

pyproject.toml

@@ -71,7 +71,7 @@ where = ["src"]
 "*" = ["*.c", "*.h", "*.pyx"]
 
 [build-system]
-requires = ["setuptools>=64", "setuptools_scm>=8", "wheel", "pkgconfig", "Cython>=3"]
+requires = ["setuptools>=64", "setuptools_scm>=8", "wheel", "pkgconfig", "Cython>=3.0.3"]
 build-backend = "setuptools.build_meta"
 
 [tool.setuptools_scm]