fix key events

Issue:
Key events where not detectable

Fix:
Add `self._pygamecanvas.grab_focus()` to math_hurdlerActivity.py

math_hurdlerActivity.py

@@ -33,6 +33,7 @@ def __init__(self, handle):
         # Note that set_canvas implicitly calls read_file when
         # resuming from the Journal.
         self.set_canvas(self._pygamecanvas)
+        self._pygamecanvas.grab_focus()
 
         # Start the game running (self.game.run is called when the
         # activity constructor returns).