docs: def exportpub(self, format="PEM"):
          """
          Returns public key as PEM or DER structure.
          """
          bio = Membio()
          if format == "PEM":
              retcode = libcrypto.PEM_write_bio_PUBKEY(bio.bio, self.key)
          else:
              retcode = libcrypto.i2d_PUBKEY_bio(bio.bio, self.key)
          if retcode == 0:
              raise PKeyError("error serializing public key")
          return str(bio)