scrobbler: allow to scrobble again

This was like completely broken and probably untested since the move to
Python 3.
Deal with file names and paths using strings instead of bytes

GLib.filename_from_utf8() returns bytes which is not really expected by
Python's own path manipulation functions and is not practical actually
anyway.
This fixes some issues with songs containing non-ASCII characters, tested
using various locales.
doc: recommends PyGObject 3.7.4 or more

3.7.4 fixes the problem from #27. It only occurs if people have playlists
with non-ASCII characters, so most people are going to be safe with a earlier
release of PyGObject. This is the recommended version anyway.
doc: more informations for the PyGObject dependency