Add __all__ for most modules beginning with 't' (#7373)

stdlib/tkinter/messagebox.pyi

@@ -1,6 +1,19 @@
+import sys
 from tkinter.commondialog import Dialog
 from typing import Any, ClassVar
 
+if sys.version_info >= (3, 9):
+    __all__ = [
+        "showinfo",
+        "showwarning",
+        "showerror",
+        "askquestion",
+        "askokcancel",
+        "askyesno",
+        "askyesnocancel",
+        "askretrycancel",
+    ]
+
 ERROR: str
 INFO: str
 QUESTION: str