text: Title: The drop table operation does not trigger the before_drop and after_drop events
  Body: **Describe the bug**
  The `table.drop` and `op.drop_table` operation does not trigger the `before_drop` and `after_drop` events.
  
  **To Reproduce**
  Create function as follow:
  ```
  @event.listens_for(Table, "before_drop")
  def record_before_event(target, conn, **kwargs):
      print("Will never be printed!")
  ```
  drop table `using table.drop()` or `op.drop_table('table')` (if migrations). The function won't be invoked.
  
  **Expected behavior**
  Table drop table should trigger an event.