message: Fix Type annotation of hybrid_decrypt_key_manager.
  The return type must be PrivateKeyManager, since the user should be able to use the public_key_data method.
diff: @@ -40,7 +40,8 @@ class _HybridDecryptCcToPyWrapper(hybrid_decrypt.HybridDecrypt):
  def from_cc_registry(
  - type_url: Text) -> key_manager.KeyManager[hybrid_decrypt.HybridDecrypt]:
  + type_url: Text
  +) -> key_manager.PrivateKeyManager[hybrid_decrypt.HybridDecrypt]:
  return key_manager.PrivateKeyManagerCcToPyWrapper(
  cc_key_manager.HybridDecryptKeyManager.from_cc_registry(type_url),
  hybrid_decrypt.HybridDecrypt, _HybridDecryptCcToPyWrapper)
  