TOKYO (Reuters) - Oil prices slipped on Tuesday amid doubts over whether OPEC and other countries will continue to restrain output, edging lower after two days of gains on U.S. economic data and hopes for a Washington-Beijing trade deal.Brent crude futures were down 14 cents, or 0.2%, at $61.99 a barrel, having gained 0.7% in the previous session.U.S. crude futures were down 18 cents, or 0.3%, at $56.36 a barrel. They gained 0.6% on Monday.Markets have been eyeing a possible extension, or even..