The patch probably needs refreshing now that first SSL contexts are in.

I wonder whether a combined boolean/string flag is really the best solution.

I think we could instead enable SNI by default and add an optional "server_hostname" to set the hostname to SSLContext.wrap_socket(), so that people can explicitly set the hostname; and otherwise take it, if possible, from the argument given to connect().

We can also add an "enable_sni" attribute to SSLContext (True by default) to allow selective disabling. This attribute would raise an exception if SNI support isn't available, which would be a way to test for it.