code: def should_build_with_cython(previous_cython_version, is_release):
      
  
      # Only build with Cython if, of course, Cython is installed, we're in a
      # development version (i.e. not release) or the Cython-generated source
      # files haven't been created yet (cython_version == 'unknown'). The latter
      # case can happen even when release is True if checking out a release tag
      # from the repository
      have_cython = False
      try:
          from Cython import __version__ as cython_version  # noqa
          have_cython = True
      except ImportError:
          pass
  
      if have_cython and (not is_release or previous_cython_version == 'unknown'):
          return cython_version
      else:
          return False
docs: Returns the previously used Cython version (or 'unknown' if not
      previously built) if Cython should be used to build extension modules from
      pyx files.