Any possible type alias that can also set a default value for a function arg?

[Matthew Carruth]

We have the `Optional[T]` type as a short-hand for Union[T | None] and telling us that said argument may not be present.

However, I find that a majority of the time, we also want to set a default value of None on the argument so that it can be evaluated without doing a getattr() check first.

iow, a lot of `foo: Optional[str] = None` in method signatures.

I'd love to see a companion to the Optional type, I'll call it Default, so that it can take a default value as a second arg, with a default of that being None. 

For example:

foo: Default[str] would be equivalent to foo: Optional[str] = None
foo: Default[str, "bar"] would be equivalent to foo: Optional[str] = "bar"

or something like that. Basically, any way to avoid writing `= None` over and over again.