Fix SF bug #1072182, problems with signed characters.

Most of these can be backported.
diff --git a/Modules/_tkinter.c b/Modules/_tkinter.c
index dd1620a..b898249 100644
--- a/Modules/_tkinter.c
+++ b/Modules/_tkinter.c
@@ -636,7 +636,7 @@
 	}
 
 	strcpy(argv0, className);
-	if (isupper((int)(argv0[0])))
+	if (isupper(Py_CHARMASK((argv0[0]))))
 		argv0[0] = tolower(argv0[0]);
 	Tcl_SetVar(v->interp, "argv0", argv0, TCL_GLOBAL_ONLY);
 	ckfree(argv0);