old_contents: import sys
  
  if 'setuptools' in sys.modules:
      try:
          from setuptools.command import build_ext as _build_ext
      except ImportError:
          # We may be in the process of importing setuptools, which tries
          # to import this.
          from distutils.command import build_ext as _build_ext
  else:
      from distutils.command import build_ext as _build_ext
  
  class build_ext(_build_ext.build_ext, object):
      def finalize_options(self):
          from Cython.Build.Dependencies import cythonize
          self.distribution.ext_modules[:] = cythonize(
              self.distribution.ext_modules)
          super(build_ext, self).finalize_options()
  
new_contents: import sys
  
  if 'setuptools' in sys.modules:
      try:
          from setuptools.command import build_ext as _build_ext
      except ImportError:
          # We may be in the process of importing setuptools, which tries
          # to import this.
          from distutils.command import build_ext as _build_ext
  else:
      from distutils.command import build_ext as _build_ext
  
  class build_ext(_build_ext.build_ext, object):
      def finalize_options(self):
          if self.distribution.ext_modules:
              from Cython.Build.Dependencies import cythonize
              self.distribution.ext_modules[:] = cythonize(
                  self.distribution.ext_modules)
          super(build_ext, self).finalize_options()
  
subject: Handle extensions=None case.