Fix sleep in dummy camera grab

time.sleep doesn't accept pint quantities, so we have to get
the magnitude of the time and then can do the sleep.
(cherry picked from commit b10bf51)

concert/devices/cameras/dummy.py

@@ -176,6 +176,6 @@ def _grab_real(self):
         to_sleep = q.count / self.frame_rate
         to_sleep = to_sleep.to_base_units() - duration * q.s
         if to_sleep > 0 * q.s:
-            time.sleep(to_sleep)
+            time.sleep(to_sleep.magnitude)
 
         return np.cast[np.uint16](tmp)