hglib: wrap all application string literals in util.b() (issue4520) Conversion also included changing use of string interpolation to string concatenation as bytes interpolation does not exist in Python 3. Indexing related to bytes was also changed to length-1 bytes through slicing as Python 3 returns an int in this instance. Tests have not been switched to using util.b() so that the change to application code can be independently verified as not being broken.