My understanding is that the example demonstrated is not what OP is looking for. In the example shown, the private key is created first and everything that follows is based on it. But the OP does not know the private key, he wants to extract the pubkey from an existing signature without knowing the private key.

@yoshimitsu777: have a look at https://pypi.org/project/secp256k1/, there is a function "recpub" which you can use comfortably and it does exactly what you are looking for. Example: