Amethyst-core

- INCOMPATIBLE: Fix serialization of set and frozenset

- INCOMPATIBLE: Remove python 2 support
- Object() is True in boolean context even if empty
- cached_property: threadsafe and add references to functools.cached_property

- WARNING: This is the last release supporting python 2
- INCOMPATIBLE: Remove Attr.__eq__ and Attr.__ne__, they aren't always what one wants
- bugfix: amethyst_inflate/amethyst_deflate are recursive
- amethyst.core.obj.AttrsMetaclass easier to subclass
- pypy added as supported interpreter in setup.py
- explicitly declare Attr() and Object() unahashable

- INCOMPATIBLE: Attr(ClassName) converters will no longer make shallow copies.
Before, obj.foo = myobject would create a new object instance.
- DEPRECATED: Object(Object) shallow copy constructor, it may end up being
preserved, but I want to break any core dependence on it.
- Attr convert parameter may take a string class name which will be auto-
loaded. Useful for parent/child attributes. E.g., parent = Attr("MyClass")
- list_of, set_of, dict_of Attr() helpers, validates a substructure.

- INCOMPATIBLE: Pass default values through validation/conversion
- Fix inflation of classes which set immutible flag in constructor
- modifiers (.strip(), and, <, ...) play well with subclassing

- Object: implement __eq__ and __ne__ (deep comparison of Attrs)