Avoid type error when reading from URL with python 3.7

This is needed on python 3.7 (at least) or you get:

Traceback (most recent call last):
  File "/data/ramp/conda3/envs/testpython37/bin/lbn-install", line 7, in <module>
    exec(compile(f.read(), __file__, 'exec'))
  File "/tmp/LbNightlyTools/scripts/lbn-install", line 14, in <module>
    sys.exit(Script().run())
  File "/data/ramp/conda3/envs/testpython37/lib/python3.7/site-packages/LbCommon/Script.py", line 107, in run
    rc = self.main()
  File "/tmp/LbNightlyTools/build/lib/LbNightlyTools/Scripts/Install.py", line 517, in main
    for subdir in listdir(url + '/packs')
  File "/tmp/LbNightlyTools/build/lib/LbNightlyTools/Scripts/Install.py", line 116, in listdir
    }[protocol](url)
  File "/tmp/LbNightlyTools/build/lib/LbNightlyTools/Scripts/Install.py", line 81, in _list_http
    parser.feed(urlopen(url).read())
  File "/data/ramp/conda3/envs/testpython37/lib/python3.7/html/parser.py", line 110, in feed
    self.rawdata = self.rawdata + data
TypeError: can only concatenate str (not "bytes") to str