Given a representation of the board, returns a list of open spaces.

def get_open_spaces(board):
  """Given a representation of the board, returns a list of open spaces."""
  open_spaces = []
  for i in range(3):
    for j in range(3):
      if board[i][j] == 0:
        open_spaces.append(encode_pos(i, j))
  return open_spaces