Cythonize is now implicit thanks to build-system.

setup.py

@@ -6,12 +6,6 @@
 
 from setuptools import Command, Extension, setup
 
-# Default to using cython, but use the .c files if it doesn't exist
-try:
-    from Cython.Build import cythonize
-except ImportError:
-    cythonize = False
-
 COMPILER_DIRECTIVES = {}
 # This Cython macro disables a build warning, obsolete with Cython>=3
 #  see : https://cython.readthedocs.io/en/latest/src/userguide/migrating_to_cy30.html#numpy-c-api
@@ -130,7 +124,7 @@ def _set_builtin(name, value):
 
 udunits_ext = Extension(
     f"{PACKAGE}._udunits2",
-    [str(Path(f"{PACKAGE}") / f"_udunits2.{'pyx' if cythonize else 'c'}")],
+    [str(Path(f"{PACKAGE}") / "_udunits2.pyx")],
     include_dirs=get_include_dirs(),
     library_dirs=library_dirs,
     libraries=["udunits2"],
@@ -140,15 +134,6 @@ def _set_builtin(name, value):
     ),
 )
 
-if cythonize:
-    [udunits_ext] = cythonize(
-        udunits_ext,
-        compiler_directives=COMPILER_DIRECTIVES,
-        # Assert python 3 source syntax: Currently required to suppress a warning,
-        #  even though this is now the default (as-of Cython v3).
-        language_level="3str",
-    )
-
 cmdclass = {"clean_cython": CleanCython, "build_ext": numpy_build_ext}
 
 kwargs = dict(