Jan 21 (Reuters) - Canada’s main stock index opened lower on Thursday, snapping a three-day winning streak, as weakness in energy and materials stocks outweighed hopes of a large fiscal stimulus package under new U.S. President Joe Biden. * At 9:33 a.m. ET (1433 GMT), the Toronto Stock Exchange’s S&P/TSX composite index was down 33.13 points, or 0.18%, at 17,981.78. (Reporting by Amal S in Bengaluru; Editing by Maju Samuel)