Implement TimerHandle.when() (GH-5473) (GH-5480)

(cherry picked from commit 3d4dbd8f019c0bbac99fc9248077044ff1039ca3)

Co-authored-by: Andrew Svetlov <andrew.svetlov@gmail.com>
diff --git a/Lib/asyncio/events.py b/Lib/asyncio/events.py
index bdefcf6..fcca5d4 100644
--- a/Lib/asyncio/events.py
+++ b/Lib/asyncio/events.py
@@ -156,6 +156,14 @@
             self._loop._timer_handle_cancelled(self)
         super().cancel()
 
+    def when(self):
+        """Return a scheduled callback time.
+
+        The time is an absolute timestamp, using the same time
+        reference as loop.time().
+        """
+        return self._when
+
 
 class AbstractServer:
     """Abstract server returned by create_server()."""