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

[iamdanfox]

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