aiohttp: Fix type of `Sec-WebSocket-Key` in `aiohttp_ws.py`

[AuroraTea]

The function binascii.b2a_base64() returns a bytes. Here needs a string Otherwise, the value of Sec-WebSocket-Key in the headers will be b'<BASE64-ENCODED_RANDOM_VALUE>'. It means there will be b' at the beginning and ' at the end.