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I believe this was added to make sure that when a MIMEText
was a subpart, the missing newline wouldn't break the
appending of the boundary text.  But we've since fixed that
breakage in another way, so I think the code in MIMEText is
wrong.  Eliminating it breaks a bunch of unit tests but they
appear easily reparable.

The b/w compat issues are tougher since there may indeed be
code relying on this.  I don't want to deprecate the whole
MIMEText class.  Maybe the least painful is to add an
argument to decide whether to add the newline or not.  I'll
bring this up on mimelib-devel and see what the consensus is.