gc.threshold() not available on all ports

gui/core/ugui.py

@@ -536,6 +536,7 @@ class Screen:
         while Screen.do_gc:
             await asyncio.sleep_ms(500)
             gc.collect()
+            if hasattr(gc, 'threshold'):
                 gc.threshold(gc.mem_free() // 4 + gc.mem_alloc())
             n += 1
             n &= 0x1F