Return types annotations when

まだ誰も着手していません。

```py
conn = await asyncpg.connect(
user=os.getenv("DB_USER"),
password=os.getenv("DB_USER_PASSWORD"),
database=os.getenv("DB_NAME"),
host=os.getenv("DB_HOST")
) # None type

conn.cursor() # None type
```

Start by tracing the public asyncpg.connect entry point and the connection object's cursor method, then inspect how their return types are exposed to Python type checkers. Done means those calls no longer appear as None in the example while preserving the existing API behavior.

索引モデルが issue の本文から書いたものです。