Add return None and some other fix

[AsfhtgkDavid]

This PR enhances type safety in asyncpg by adding type hints to several methods in the Pool class. The changes improve code readability, facilitate static analysis, and help developers catch potential type-related issues early. Changes:

• Added type annotations to selected Pool methods.
• Ensured compatibility with existing function signatures.
• Verified correctness with type checkers (e.g., mypy).

These improvements align with the ongoing effort to introduce type hints across the codebase.