After bumping into r.david.murray in the elevator I got the impression setting the bufsize argument to the Popen call would be a better idea.

I found that BufferedReader/Writer were using a DEFAULT_BUFFER_SIZE set somewhere in the c part of io. To cut down on magic numbers, this imaplib patch imports that constant and uses it on the Popen call. 

It doesn't seem to introduce test failures and still fixes the imap desynchronization problem seen at the porting clinic.