PEP8 cleanup for node/market.py through node/openbazaar.py

Minor PEP8 clean up commit 2 of 3.
This won't look good split in 2 statements, so here's an alternative:
```python
print (
    "[openbazaar]"
    "MarketApplication.clean_upnp_port_mapping() failed!"
)
```
Python auto-joins successive strings, even when separates by line breaks.