pySCG: Either add is() to CWE-595 or create a new rule

While creating https://github.com/ossf/wg-best-practices-os-developers/tree/main/docs/Secure-Coding-Guide-for-Python/CWE-697/CWE-595 we found there is a need to explain why we need to distinguish between `==` and `is()`.  CWE-595 does not explain is().

Either add is() to CWE-595  or create a new rule.