[3.15] gh-88574: Do not swallow the line after a terminating literal in imaplib ...

[3.15] gh-88574: Do not swallow the line after a terminating literal in imaplib (GH-153317) (GH-153324)

GH-152751 skipped a spurious blank line after a literal unconditionally,
corrupting a response that ends with a literal (such as a mailbox name
returned by LIST): its empty trailer was mistaken for the blank and the
following line was swallowed.

The blank is now skipped only inside an unclosed parenthesis.  After a
literal that ends the response it instead arrives before the next
response and is skipped there.
(cherry picked from commit 6b81784f57c6e7ddef61c8db26bb896b3633dcae)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>