Fix annotation of decrypted_secret

[marinelay]

Close #4198
Fix type annotation of self._decrypted_secret from str to bytes

[Bibo-Joshi]

Thanks for the PR! Just one question: Is the "# type: ignore" iin line 183 still necessary with your changes or can it be removed?

[marinelay]

I think it is still necessary despite the change.
The type checker infers self.get_bot().private_key can be None, so it reports false alarm with the message "None does not have attribute 'private_key".
I used Pyright for this, but I believe that mypy is the same.

[Bibo-Joshi]

Ah, right … Then let's merge. Thanks for the contribution!