code: def key_usage(self):
          
          try:
              ext = self.x509.extensions.get_extension_for_oid(ExtensionOID.KEY_USAGE)
          except x509.ExtensionNotFound:
              return None
          return KeyUsage(ext)
docs: The :py:class:`~django_ca.extensions.KeyUsage` extension, or ``None`` if it doesn't exist.