2.2.0 - Subclassing & speed-ups

Added

• Support PEP 517.

Changed

• Some internal changes to ease subclassing Interval(see #58):
  • Use self.__class__ instead of Interval to create new instances;
  • Deprecate and move mergeable function to Interval._mergeable class method;
  • Interval.from_atomic is now a class method instead of a static method.
• Speed up lookups in IntervalDict (#65, Jeff Trull).
• Speed up removals in IntervalDict.
• Speed up intersection for non-overlapping intervals (#66, Jeff Trull).
• Speed up .overlaps and .contains for non-overlapping intervals/items.