GHSA-VXJ7-4XRP-5VR4 aiosmtplib: STARTTLS response injection

Impact When a connection is upgraded with STARTTLS, aiosmtplib reads the server's 220 go-ahead reply and immediately performs the TLS handshake without discarding any data still sitting in the receive buffer. Bytes the protocol read off the plaintext socket before the handshake survive across the...