test_tkinter: use a context manager to close directly the pipe

Patch written by Nadeem Vawda
diff --git a/Misc/ACKS b/Misc/ACKS
index 6a3a25b..dccb779 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -866,6 +866,7 @@
 Atul Varma
 Dmitry Vasiliev
 Alexandre Vassalotti
+Nadeem Vawda
 Frank Vercruesse
 Mike Verdone
 Jaap Vermeulen