[Python-ideas] Add an optional type file for Type Annotation

[李者璈]

I'm inspired by TypeScript.

TypeScript allows people to add **.d.ts* *to annotate ECMAScript's type. So 
this can expand the TypeScript's scenes to be used。

So I think can we add an optional mechanism to allow people add the extra 
file to annotate the existing code.

Type Annotation can be used after Python 3.5. but many lib/framework has to 
be compatible for the Python 3.0-3.4, they can't use annotation.

If we have this mechanism, the lib/framework can continue to focus on the 
function, and the third group can add the extra type annotation for 
lib/framework

I think it should be OK

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