[3.9] bpo-20028: Improve error message of csv.Dialect when initializing (GH-28705)#28831

miss-islington merged 1 commit intopython:3.9python/cpython:3.9from

miss-islington:backport-34bbc87-3.9miss-islington/cpython:backport-34bbc87-3.9Copy head branch name to clipboard