MILWAUKEE — Law enforcement is tightening down security in Milwaukee in advance of the Republican National Convention following the assassination attempt on its presumed nominee Donald Trump at a campaign rally in Pennsylvania on Saturday. Restrictions on pedestrian access to a 20-block area surrounding Fiserv Forum had been set to go into effect at 6 p.m. central time Sunday under the operational security plan developed by the Secret Service, but authorities moved up implementation of the pedestrian security restrictions. By noon on Sunday, residents and convention vendors were greeted with 1…