gh-124111: Fix TCL 9 thread detection (GH-128103)

Misc/NEWS.d/next/Library/2025-11-12-15-42-47.gh-issue-124111.hTw4OE.rst

@@ -0,0 +1,2 @@
+Updated Tcl threading configuration in :mod:`_tkinter` to assume that
+threads are always available in Tcl 9 and later.

Modules/_tkinter.c

@@ -575,8 +575,12 @@ Tkapp_New(const char *screenName, const char *className,
 
     v->interp = Tcl_CreateInterp();
     v->wantobjects = wantobjects;
+#if TCL_MAJOR_VERSION >= 9
+    v->threaded = 1;
+#else
     v->threaded = Tcl_GetVar2Ex(v->interp, "tcl_platform", "threaded",
                                 TCL_GLOBAL_ONLY) != NULL;
+#endif
     v->thread_id = Tcl_GetCurrentThread();
     v->dispatching = 0;
     v->trace = NULL;