I'm assigning this one to me.
Btw, before writing anything I think it would be better to decide what to do first, possibly by adopting the same approach everywhere (smtplib, imaplib, poplib, urllib and http.client).

My proposal: 

- the user should still be able to use keyfile and certfile if desired, they're quicker to use and backward compatibility must be maintained.

- SSL context can be provided by passing a new "context" argument to the constructor.

- if context arg is passed FTP_TLS.ssl_version should be ignored and SSLContext.protocol used instead

- keyfile/certfile and context arguments should be mutually exclusive (ValueError)


Makes sense?