Typing: Correctly set types when getting `Record` fields

还没有人认领这个 Issue。

Consider the following:

```python
import asyncio

import asyncpg

async def main() -> int:
connection_string = "postgresql://postgres:postgres@127.0.0.1:5432/postgres"
conn = await asyncpg.connect(connection_string)
result = await conn.fetch("SELECT 1 + 1 AS num", record_class=Row)
row = result[0]
num = row["num"]
print(type(num))
return num # mypy error: Returning Any from function declared to return "int"

class Row(asyncpg.Record):
num: int

if __name__ == "__main__":
asyncio.run(main())
```

In essence, the type of `num` is `Any` instead of `int`. I tried to make `Row` inherit from `TypedDict` or use a `dataclass`, both failed.
I'll also be happy to help develop this feature with some general guidance.

Start with the conn.fetch(..., record_class=Row) example and the asyncpg.Record field access shown in the issue. Investigate how the declared Row.num annotation could affect the inferred type of row["num"]. Done means the example's num is inferred as int and no longer produces mypy's Any return error.

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