including:
- simplify type hierarchy, merging the frozen types into the base types
- upgrade to latest hypothesis (3.38.0)
- use Reversible for order-sensitive FrozenOrderedBidict.__eq__
- fix bug where bidict._on_dup_kv was not set to ON_DUP_VAL as documented
- drop leading underscore from attrs that should be public
- improve setup.py/metadata
- rename DuplicationBehavior -> DuplicationPolicy
- drop ON_DUP_VAL duplication policy in preference to just using None
- reduce ordered bidict memory usage and speed up copy