[#706] PEP8 Compliance

If you want me to actually remove the changes from `dht.py`, `market.py` and `transport.py` let me know.
I can also add a test for PEP8 compliance.
why the new variable? what was the PEP8 complain there? (the line didn't seem that long)
and in the end `has_valid_signature` isn't used anywhere. The method signature seems self explanatory.