GHSA-5RVQ-CXJ2-64VF python-multipart: Quadratic-time querystring parsing with semicolon separators causes CPU denial of service

Summary When parsing application/x-www-form-urlencoded bodies, QuerystringParser located the field separator with a two step lookup: it first scanned the entire remaining buffer for &, and only when no & existed anywhere ahead did it fall back to scanning for ;. For a body that uses ; as the...