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

(cherry picked from commit 2fb797e93c6bbd44dfcbe23f63acfa240a87e48a)

Co-authored-by: Thomas Klausner <tk@giga.or.at>
diff --git a/Misc/NEWS.d/next/Core and Builtins/2021-12-07-11-42-44.bpo-46000.v_ru3k.rst b/Misc/NEWS.d/next/Core and Builtins/2021-12-07-11-42-44.bpo-46000.v_ru3k.rst
new file mode 100644
index 0000000..68e4bfa
--- /dev/null
+++ b/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.
diff --git a/Modules/_cursesmodule.c b/Modules/_cursesmodule.c
index 511073f..a1de435 100644
--- a/Modules/_cursesmodule.c
+++ b/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);