Skip builtin byte payloads in transport metadata

[keilogic]

Summary

• treat builtin str, bytes, and bytearray payloads as non-transport data in TransportMeta
• avoid assigning metadata fields onto immutable raw byte payloads
• clean the touched file's stale unused import and local spacing issues so lint passes

Tests

• python -m py_compile chains\links\transport_meta.py
• python -m flake8 chains\links\transport_meta.py --max-line-length=140
• fake-dpkt runtime probe: an input item with packet.data == b"raw payload" is yielded unchanged with transport is None, without TypeError: 'bytes' object does not support item assignment
• git diff --check

Fixes #10