Update the signature of decorator.decorator (#3336)

Nothing prevents a decorator defined using `@decorator` from
changing the signature of the decorated function. For example,
this example changes the return type to `str`:

```
from decorator import decorator

@decorator
def stringify(f, *args, **kwargs) -> str:
    return str(f(*args, **kwargs))
```

The old signature caused false positives in internal Dropbox code.

I couldn't come up with a signature that would produce better types
with mypy while not generating false positives.

third_party/2and3/decorator.pyi

@@ -75,7 +75,7 @@ class FunctionMaker(object):
     ) -> Callable[..., Any]: ...
 
 def decorate(func: _Func, caller: Callable[..., Any], extras: Any = ...) -> _Func: ...
-def decorator(caller: Callable[..., Any], _func: Optional[Callable[..., Any]] = ...) -> Callable[[_C], _C]: ...
+def decorator(caller: Callable[..., Any], _func: Optional[Callable[..., Any]] = ...) -> Callable[[Callable[..., Any]], Callable[..., Any]]: ...
 
 class ContextManager(_GeneratorContextManager[_T]):
     def __call__(self, func: _C) -> _C: ...