PEP8 openbazaar.py through ws.py

PEP8 clean up 3 of 3.  Orders.py has some very deeply nested dictionaries which necessitate long lines.  This situation may suggest minor future refactoring of the affected methods.
No need to use backslash or join strings. Use formatting:
```python
offer_data_json = '{"Seller": {%s' % (
    offer_data[:index_of_seller_signature - 2]
)
```