Dec 29 (Reuters) - Canada’s main stock index opened higher on Tuesday, after a long holiday weekend, helped by gains in heavyweight energy stocks and the approval of a $2.3 trillion coronavirus aid package in the United States. * At 09:31 a.m. ET (1431 GMT), the Toronto Stock Exchange’s S&P/TSX composite index was up 31.39 points, or 0.18%, at 17,655.27.