adding a stub for tkinter.messagebox (#2113)

stdlib/3/tkinter/messagebox.pyi

@@ -0,0 +1,31 @@
+from tkinter.commondialog import Dialog
+from typing import Any, Optional
+
+ERROR: str
+INFO: str
+QUESTION: str
+WARNING: str
+ABORTRETRYIGNORE: str
+OK: str
+OKCANCEL: str
+RETRYCANCEL: str
+YESNO: str
+YESNOCANCEL: str
+ABORT: str
+RETRY: str
+IGNORE: str
+CANCEL: str
+YES: str
+NO: str
+
+class Message(Dialog):
+    command: str = ...
+
+def showinfo(title: Optional[str] = ..., message: Optional[str] = ..., **options: Any) -> str: ...
+def showwarning(title: Optional[str] = ..., message: Optional[str] = ..., **options: Any) -> str: ...
+def showerror(title: Optional[str] = ..., message: Optional[str] = ..., **options: Any) -> str: ...
+def askquestion(title: Optional[str] = ..., message: Optional[str] = ..., **options: Any) -> str: ...
+def askokcancel(title: Optional[str] = ..., message: Optional[str] = ..., **options: Any) -> bool: ...
+def askyesno(title: Optional[str] = ..., message: Optional[str] = ..., **options: Any) -> bool: ...
+def askyesnocancel(title: Optional[str] = ..., message: Optional[str] = ..., **options: Any) -> Optional[bool]: ...
+def askretrycancel(title: Optional[str] = ..., message: Optional[str] = ..., **options: Any) -> bool: ...