PR: Destroy tk app to avoid memory leak (#1008)

Co-authored-by: Quentin Peter <quentin.peter@bluewin.ch>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

ipykernel/eventloops.py

@@ -247,6 +247,8 @@ def process_stream_events(stream, *a, **kw):
             if stream.flush(limit=1):
                 app.tk.deletefilehandler(stream.getsockopt(zmq.FD))
                 app.quit()
+                app.destroy()
+                del kernel.app_wrapper
 
         # For Tkinter, we create a Tk object and call its withdraw method.
         kernel.app_wrapper = BasicAppWrapper(app)
@@ -297,7 +299,8 @@ def start(self):
 def loop_tk_exit(kernel):
     try:
         kernel.app_wrapper.app.destroy()
-    except RuntimeError:
+        del kernel.app_wrapper
+    except (RuntimeError, AttributeError):
         pass