After nearly a year of next-to-no deal-making, cannabis companies are gearing up for mergers and acquisitions as realistic stock valuations and the prospect of U.S. legalization attract buyers to a sector that has been decimated by oversupply and other issues, executives and investors say. Profitable cannabis companies want to buy their way into niche segments and expand their brands, betting that the November U.S. presidential election will lead to weed becoming legal across the United…