support for restrictions, tests

[beckjake]

Allow for setting a restrict metadata member that restricts enums to a subset of a given enum. This will (I hope...) help hologram/jsonschema play nicely mypy, and obviate the need for StrLiteral anywhere (which doesn't pickle, causing Windows code involving multiprocessing.Process to fail in surprising and annoying ways).

Now you can define types as follows, and both mypy and hologram will be happy:

class MySelector(StrEnum):
    A = 'a'
    B = 'b'
    C = 'c'

@dataclass
class FancyRestrictBase(JsonSchemaMixin):
    foo: MySelector

@dataclass
class FancyRestrictATrue(FancyRestrictBase):
    foo: MySelector = field(metadata={'restrict': [MySelector.A]})
    is_something: bool = field(metadata={'restrict': [True]})
    bar: str

@dataclass
class FancyRestrictAFalse(FancyRestrictBase):
    foo: MySelector = field(metadata={'restrict': [MySelector.A]})
    is_something: bool = field(metadata={'restrict': [False]})
    bar: str

@dataclass
class FancyRestrictBC(FancyRestrictBase):
    foo: MySelector = field(metadata={'restrict': [MySelector.B, MySelector.C]})
    bar: str

@dataclass
class HasFancyRestricted(JsonSchemaMixin):
    thing: Union[FancyRestrictATrue, FancyRestrictAFalse, FancyRestrictBC]