`@trait` has special meaning to the experimental mypyc compiler,
slightly restricting the behavior of the class in exchange for
allowing it to be multiply inherited from.
add_argument's type argument was recently changed to be (approximately)
`Callable[[_Text], _T]`. Because of contravariant subtyping for functions,
this had the effect of requiring that add_argument *always*
be unicode, which is wrong.
Change it to be `Callable[[str], _T]`.
* Make {C,WIN,PY}FUNCTYPE's restype argument optional (for void returns)
* Make WinError return WindowsError
* Don't have Array explicitly inherit from Sized
(since inheriting from classes with different metaclasses causes problems,
and it is a protocol anyways)
* Have Array.__iter__ properly return an Iterator instead of an Iterable
Michael J. Sullivan