Better type annotation for CORSMiddleware?

[roy-work]

See: https://github.com/encode/starlette/blob/master/starlette/middleware/cors.py#L19C1-L19C50

(I'll be discussing allow_origins, but the logic herein should apply to allow_methods, too.)

CORSMiddleware defines the allow_origins parameter as,

        allow_origins: typing.Sequence[str] = (),
        allow_methods: typing.Sequence[str] = ("GET",),

But the only thing we ever do with allow_origins is … in self.allow_origins, to test if an origin is allowed. This type annotation, then, could be just typing.Container[str]; Sequence is an ordered collection, i.e., it requires indexing. This means Sequence permits pretty much only list & tuple, but it would make a lot of sense (and be faster¹) to pass something like a frozenset here. (But a frozenset is not a Sequence.)

_{¹frozenset __contains__ checks are about 5× faster on my app's current origin list. But the absolute difference isn't huge: it's 62ns for a list, vs. 11ns for a set, which isn't much in the grand scope of processing an HTTP request. It is more that a set makes more semantic sense to be passable here.}

[Kludex]

Okay. PR welcome.