Magic methods usually don’t have docstrings, since they implement one operation that is described in one place (think __len__ vs. len). You could remove the doc of __enter__ and turn the doc of __exit__ into a comment or move it to the reST file when you say that FTP supports the context management protocol.

What did you add a cmd_noop method that does the same thing as cmd_user for?

Will the change from handler to handler_instance not break third-party code?