GHSA-836r-79rf-4m37: Soup Sieve: Regular Expression Denial of Service (ReDoS) via Selector Parser

The soupsieve library (a CSS selector engine used by Beautiful Soup 4) contains a ReDoS vulnerability (regular expression denial of service, where a maliciously crafted input causes the regex engine to hang by repeatedly backtracking). When the CSS parser encounters an unterminated quoted attribute selector like `[a="xxxx...`, the regex pattern enters catastrophic backtracking, and just 300 bytes of input can cause the parser to hang for over 3 seconds, freezing any application that uses soupsieve.compile() or Beautiful Soup's .select() methods with untrusted input.