GHSA-6JV3-5F52-599M python-multipart: Semicolon treated as querystring field separator enables parameter smuggling

Summary QuerystringParser treated ; as a field separator in application/x-www-form-urlencoded bodies, in addition to &. The WHATWG URL standard, modern browsers, and Python's urllib.parse since the CVE-2021-23336 fix treat only & as a separator. This creates a parser differential: the same bytes...