Add PEP 613 TypeAlias stub to typing_extensions

[east825]

At the moment this special marker annotation described in
PEP 613 exists only in typing_extensions module
(see python/typing#732).

[east825]

BTW, what's the difference between declarations such as

Any = object()

TypedDict: object

(in typing.pyi)

and

TypedDict: object = ...

(in typing_extensions.pyi)

These are considered equivalent, aren't they? Which one is preferred then?

[srittau]

Foo: object is the preferred form. The = ... is redundant. Any = object() doesn't make much sense in stubs. The reason that type checkers don't complain is that typing.pyi is usually special-cased by them.

[east825]

Ok, thanks! I'll drop "= ..." part in my PR then.