GeezerVison

…r an inconsistent guessed dialect

csv.Sniffer.sniff could return a dialect whose delimiter equaled its quote character. Using that dialect (directly, or via csv.Sniffer.has_header, which builds a reader from the sniffed dialect) leaked a raw ValueError from the _csv extension instead of the module's own csv.Error. sniff now validates its guessed dialect before returning it and re-raises the _csv ValueError as csv.Error, so both sniff and has_header report the module exception type. The direct csv.reader and csv.writer paths and the Dialect._validate TypeError/ValueError behavior are unchanged.