asyncio.StreamWriter.transport not specific enough

[Akuli]

This works at runtime:

import asyncio

async def handle_connection(reader: asyncio.StreamReader, writer: asyncio.StreamWriter) -> None:
    print(writer.transport.get_write_buffer_size())
    writer.write(b"x" * (256*1024*1024))
    print(writer.transport.get_write_buffer_size())

async def main() -> None:
    async with await asyncio.start_server(handle_connection, port=1234) as server:
        await server.serve_forever()

asyncio.run(main())

but doesn't type check:

/tmp/echo2.py:4: error: "BaseTransport" has no attribute "get_write_buffer_size"
/tmp/echo2.py:6: error: "BaseTransport" has no attribute "get_write_buffer_size"

According to asyncio docs, a WriteTransport has a get_write_buffer_size() method, and guessing from the names of things, I think writer.transport should always be a WriteTransport.