start_tls() loses buffered data and doesn't work when upgrading a stream

When using StreamWriter.start_tls() (or loop.start_tls()) to upgrade a connection to TLS mid-stream, any data already buffered in the StreamReader is lost, and the call hangs forever.

The problem is present in asyncio too: https://github.com/python/cpython/issues/142352 , and a solution has been provided: https://github.com/python/cpython/pull/142354