[#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.
oh that's even better @Renelvon :+1: 

@prikhi if you still feel like renaming the signature variable to something else, I'd recommend `hexsig` to explicilty denote this is the hex encoded representation.

```python
hexsig = message['sig'].decode('hex')
signed_data = message['data']

if self.validate_signature(hexsig, signed_data):
...
```
tiny shorter and self documenting.