v2.0.0

Breaking Changes

• Drop support for Python 2.7, 3.4 & 3.5
• When multiplying bags, the cartesian product creates a tuple instead of adding
  the elements.
• bags no longer inherit from Set
  • can no longer compare as equal to Sets
• Rename and expose bag and set base classes
  • _basebag -> Bag
  • _basesetlist -> SetList

Added

• Added IndexedDict
• Improve efficiency for large bag operations
• Add setlist.swap
• Add bag.count, CountsView & UniqueElementsView
• Add bag.issubset and issuperset
• Add support for Python 3.8 & 3.9
• Add Sentinel
• Make MappedRange a class instead of a namedtuple
• Add change log