Transport code calculation does not use same guardrails as official firmware

Hi,

fyi - I filed an issue in [MeshCore](https://github.com/meshcore-dev/MeshCore/issues/2434) as I uncovered that [your implementation](https://github.com/rightup/pyMC_core/blob/main/src/pymc_core/protocol/transport_keys.py#L14) of transport code normalization differs seriously from the official firmware.