obspy.seedling Python 2/3 compatibility

The futurize conversion done in 38f5a71b02113c0ba5022ab5a418612fd205a67f broke some SeedLink functionality.

The following code triggers an error:

```python
from obspy.seedlink.slclient import SLClient

client = SLClient(loglevel='DEBUG')
client.slconn.setSLAddress('geofon.gfz-potsdam.de:18000')
client.initialize()

client.slconn.requestInfo('STATIONS')
client.run()
```

The same error is triggered when running the (manual) ``obspy.seedlink`` tests.

The ``SeedLinkConnection`` makes use of bytearrays and Python 2 strings (str).

I tried fixing this in c4cc8c8bdb0398249d6989da7204accfce39a5ab. The fix will disregard non-ASCII characters in INFO responses though. While SeedLink is primarily an ASCII protocol, I am unsure if the XML returned in INFO responses may in principle contain non-ASCII characters.

Also I currently have no Python 3 environment to test this.