Improve authentication.requires typings

[sobolevn]

I've noticed that authentication.requires does not preserve type information about the passed function.

After applying authentication.requires on any function, it changes its type to Any.
Which is problematic: Any is not really safe to work with.

So, I propose to change that. From what I can tell, @requires does preserve the original function's signature. So, a bound typevar should be enough.

from starlette.authentication import requires
from starlette.responses import PlainTextResponse
from starlette.requests import Request


@requires(['authenticated', 'admin'])
async def dashboard(request: Request) -> PlainTextResponse:
    return PlainTextResponse('Hello, world!')

reveal_type(dashboard)
# Before:
# note: Revealed type is "Any"

# After:
# note: Revealed type is "def (request: Request) -> typing.Coroutine[Any, Any, PlainTextResponse]"

Reference PR: #1659

[Kludex]

The PR was accepted.