Update imaplib2 to 2.37

The only fix is
 - add missing idle_lock in _handler().

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>
Imaplib2: trade backticks to repr()

Backticks are gone in Python 3.x.

GitHub issue: https://github.com/OfflineIMAP/offlineimap/issues/121
Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>