python 3 needs file opned for text, not binary/bytes

tested with:    with open('/etc/hostname', 'wt') as f:

reactive/ubuntu.py

@@ -19,7 +19,7 @@ def update_hostname():
     if not hostname:
         return
 
-    with open('/etc/hostname', 'wb') as f:
+    with open('/etc/hostname', 'w') as f:
         f.write(hostname)
 
     subprocess.check_call(['hostname', hostname])