Drop handshake_done and peer_cert members from PySSLSocket struct. The
peer certificate can be acquired from *SSL directly.
SSL_get_peer_certificate() does not trigger any network activity.
Instead of manually tracking the handshake state, simply use
SSL_is_init_finished().

In combination these changes fix auto-handshake for non-blocking
MemoryBIO connections.
Signed-off-by: Christian Heimes <christian@python.org>