'Start a TLS session on the active connection as specified in RFC 2595.
context - a ssl.SSLContext'

def stls(self, context=None):

    if (not HAVE_SSL):
        raise error_proto('-ERR    TLS    support    missing')
    if self._tls_established:
        raise error_proto('-ERR    TLS    session    already    established')
    caps = self.capa()
    if (not ('STLS' in caps)):
        raise error_proto('-ERR    STLS    not    supported    by    server')
    if (context is None):
        context = ssl._create_stdlib_context()
    resp = self._shortcmd('STLS')
    server_hostname = (self.host if ssl.HAS_SNI else None)
    self.sock = context.wrap_socket(self.sock, server_hostname=server_hostname)
    self.file = self.sock.makefile('rb')
    self._tls_established = True
    return resp
