Generate a real Python 'set' for the Conjure set<T> type

What's happening

Given the Conjure definition:

SetExample:
  fields:
    items: set<string>
    doubleItems: set<double>

conjure-python 3.9.0 doesn't actually enforce uniqueness of the items field because it generates a list:

class SetExample(ConjureBeanType):

    @classmethod
    def _fields(cls):
        # type: () -> Dict[str, ConjureFieldDefinition]
        return {
            'items': ConjureFieldDefinition('items', ListType(str)),
            'double_items': ConjureFieldDefinition('doubleItems', ListType(float))
        }

    _items = None # type: List[str]
    _double_items = None # type: List[float]

    def __init__(self, double_items, items):
        # type: (List[float], List[str]) -> None
        self._items = items
        self._double_items = double_items

    @property
    def items(self):
        # type: () -> List[str]
        return self._items

    @property
    def double_items(self):
        # type: () -> List[float]
        return self._double_items

What should happen

We should codegen a python type that actually enforces uniqueness, e.g. set or frozenset: https://docs.python.org/2/library/stdtypes.html#set

Start by tracing the generator that emits the shown SetExample fields and ListType annotations. Update the generated representation for set and set to enforce uniqueness, then verify the generated properties and type annotations reflect that set-like behavior.