Update issues.py

[itsmenick212]

1. Protocol Anomalies (PROTO Opcode): Detects multiple or misplaced PROTO opcodes, which could indicate file tampering or incorrect usage. It flags duplicate PROTO opcodes or PROTO not being the first opcode for higher protocol versions.
2. Unused Variables: Flags variables that are serialized but never used, which can point to inefficient serialization or possibly hidden, unnecessary data.
3. Invalid Opcodes: Identifies opcodes that are not recognized as valid, signaling that the pickle file might be corrupted or tampered with.
4. Unsafe Loaders: Detects the use of potentially dangerous functions (like pickle.loads()) that could execute arbitrary code, preventing security risks in untrusted pickle files.