By Kayla Tarnowski and Carlos Osorio WINDSOR, Ontario (Reuters) – A standoff between Canadian police and protesters blocking a key bridge to the United States continued on Saturday, more than seven hours after authorities moved in seeking to end the blockade of the important trade corridor. Demonstrators opposing the government’s strict pandemic restrictions have occupied the Ambassador Bridge for the fifth straight day, snarling international trade and prompting President Joe Biden to call for an end to the siege. But there was still no sign when traffic would resume. While police have succes…