stubgen: Use Generator ... instead of Generator ..., None, None

stubgen: Use Generator ... instead of Generator ..., None, None: a task in MiMo-V2.6-RL-oss: Agentic RL Environments (MiMo RL release). stubgen: Use Generator ... instead of Generator ..., None, None Typeshed now uses default of None for the second and third argument to Generator and we've started…