test_ftplib now passes, but it only tests socket timeouts...
My corrections were based on runs like:
python -m ftplib ftp.gnu.org -l emacs

Note: In this kind of code, byte objects make the code more difficult:
- it is easy to forget the b prefix 
eg:    resp[:3] in (b'250', '200')
- getitem does not return a single byte:
eg:    resp[0] == b'2'

Or should ftplib decode all the lines it receives?