(Well, actually the version that fixes the svn.python.org certificate
problem, as of https://github.com/python/cpython/commit/e5b4fee762a5f4954912e4cbde0ba113b687a66f.)

This showed several errors in our SSLSocket impleentation:

- read/write/do_handshake raise the correct error for unconnected
  sockets.
- SSLSocket.unwrap returns a working socket object
- SSLSocket allows changing the context
- SNI callbacks get a working SSLSocket that can change the context