Sync recent typing and typing_extensions updates (#3070)

This includes two things to sync up with recent runtime updates:
* Move `Final`, `@final`, `Literal`, and `TypedDict` to `typing` (`typing_extensions` still defines or re-exports them)
* Rename `@typing.runtime` to `@typing.runtime_checkable`, while keeping `@runtime` as a backwards-compatible alias in `typing_extensions`.

third_party/2and3/typing_extensions.pyi

@@ -19,7 +19,9 @@ _F = TypeVar('_F', bound=Callable[..., Any])
 _TC = TypeVar('_TC', bound=Type[object])
 class _SpecialForm:
     def __getitem__(self, typeargs: Any) -> Any: ...
-def runtime(cls: _TC) -> _TC: ...
+def runtime_checkable(cls: _TC) -> _TC: ...
+# This alias for above is kept here for backwards compatibility.
+runtime = runtime_checkable
 Protocol: _SpecialForm = ...
 Final: _SpecialForm = ...
 def final(f: _F) -> _F: ...