This is a straightforward implementation of client-side use of SSL, but
it's missing a test case for evaluation.  It should include a patch to
test_ftplib to test it.

Another thing to look at is what the useful arguments are to pass in for
TLS usage over FTP.  If, for example, the client needs to validate the
server's certificate or identity, provision should be made for a file of
cacerts to be passed to the FTP_TLS instance.  Passing in a keyfile and
certfile is usually only necessary when the client uses them to identify
itself to the server.