This avoids circular dependency happening when importing MAGIC_BYTES.
Before,
p2p.py <--import for EncryptedP2PState-- v2_p2p.py
| ^
| |
└---------import for MAGIC_BYTES----------┘
Now, MAGIC_BYTES are kept separately in messages.py
Co-authored-by: Martin Zumsande <mzumsande@gmail.com>