Prevent doctest from leaking file into current working directory.

Now that the special case for DOCTEST_MODE in Graphics.show is gone,
we have to be more careful about where we place our output files.

src/sage/plot/animate.py

@@ -461,7 +461,8 @@ def graphics_array(self, ncols=3):
 
             sage: g = a.graphics_array(ncols=2); print g
             Graphics Array of size 2 x 2
-            sage: g.show('sage.png') # optional
+            sage: f = sage.misc.temporary_file.tmp_filename(ext='.png')
+            sage: g.show(f) # optional
 
         Frames can be specified as a generator too; it is internally converted to a list::