Add default timeout for Condition.wait_for (#1726)

Fixes:

- Too few arguments for "wait_for" of "Condition"

when timeout is not specified.

stdlib/2and3/threading.pyi

@@ -123,7 +123,7 @@ class Condition:
     def wait(self, timeout: Optional[float] = ...) -> bool: ...
     if sys.version_info >= (3,):
         def wait_for(self, predicate: Callable[[], _T],
-                     timeout: Optional[float]) -> _T: ...
+                     timeout: Optional[float] = ...) -> _T: ...
     def notify(self, n: int = ...) -> None: ...
     def notify_all(self) -> None: ...
     def notifyAll(self) -> None: ...