#21815: violate IMAP RFC to be compatible with, e.g., gmail
and others, including imaplib's own behavior. I'm applying this only to 3.6
because there's a potential backward compatibility concern: if there are
servers that include ] characters in the 'text' portion of their imap
responses, this code change could introduce a new bug.
Patch by Lita Cho, reviewed by Jessica McKellar, Berker Peksag, Maciej Szulik,
silentghost, and me (I fleshed out the comments with the additional
info/concerns.)