Close Requests.Session when done
Close Requests.Session when done

When we are done with requests.Session() we should close it.

This way we can silence all the socket warnings:

```
  /home/lefteris/.virtualenvs/raidenpy3/lib/python3.6/site-packages/yaml/error.py:7: ResourceWarning: unclosed <socket object, fd=43, family=2, type=2049, proto=6>
    self.name = name
  /home/lefteris/.virtualenvs/raidenpy3/lib/python3.6/site-packages/yaml/error.py:7: ResourceWarning: unclosed <socket object, fd=35, family=2, type=2049, proto=6>
    self.name = name
  /home/lefteris/.virtualenvs/raidenpy3/lib/python3.6/site-packages/yaml/error.py:7: ResourceWarning: unclosed <socket object, fd=41, family=2, type=2049, proto=6>
    self.name = name
```

hackaugusto/raiden,hackaugusto/raiden