WASHINGTON — The Trump administration is set to announce a policy change allowing year-round sales of gasoline with higher ethanol blends, a move that could bolster Midwest Republicans in tough election contests and appease corn farmers battered by agricultural tariffs, according to people briefed on the planned announcement.President Donald Trump is slated to unveil the new policy in coming weeks, possibly in Iowa, where farmers, biofuel producers and politicians have been clamoring for the move that’s likely to expand the U.S.