After digging a bit further it seems EAGAIN occurs in case a timeout was previously set against the socket as in ftplib.FTP(..., timeout=2) (at least on Linux, FWICT).
As such, we can debate whether avoid using select/poll if timeout was not set.
I'll that a look at the man pages of the other POSIX platforms and figure whether EAGAIN is interpreted as on Linux.
Other than that, the patch is reasonably ok to me and can be committed as-is and blocksize argument tuning can be discussed in a separate ticket.