bpo-46000: Improve NetBSD curses compatibility (GH-29947)

(cherry picked from commit 2fb797e93c)

Co-authored-by: Thomas Klausner <tk@giga.or.at>

Misc/NEWS.d/next/Core and Builtins/2021-12-07-11-42-44.bpo-46000.v_ru3k.rst

@@ -0,0 +1 @@
+Improve compatibility of the :mod:`curses` module with NetBSD curses.

Modules/_cursesmodule.c

@@ -1226,8 +1226,8 @@ PyCursesWindow_ChgAt(PyCursesWindowObject *self, PyObject *args)
         return NULL;
     }
 
-    color = (short)((attr >> 8) & 0xff);
-    attr = attr - (color << 8);
+    color = (short) PAIR_NUMBER(attr);
+    attr = attr & A_ATTRIBUTES;
 
     if (use_xy) {
         rtn = mvwchgat(self->win,y,x,num,attr,color,NULL);