Hot fix for #38, #39 and small changes to support #37

Added

• IntervalDict supports | and |=, the same way dict will do starting from Python 3.9 (#37).

Fixed

• Fix invalid simplification of 3+ intervals when a closed interval shares the lower bound of an open one (#38).
• Fix the order in which items are returned from an IntervalDict when a closed interval shares the lower bound of an open one (#39).