cerbero: Fix TypeError on older Python versions
cerbero: Fix TypeError on older Python versions

open() does not accept pathlib.Path objects in Python 3.5. Use the
.open() method instead which works on all supported Python versions.
Fixes:

TypeError: invalid file: PosixPath('[...]/README-LICENSE-INFO.txt')

nirbheek/cerbero,GStreamer/cerbero,GStreamer/cerbero,atsushieno/cerbero,fluendo/cerbero,fluendo/cerbero,atsushieno/cerbero,fluendo/cerbero,nirbheek/cerbero,nirbheek/cerbero,atsushieno/cerbero,fluendo/cerbero,fluendo/cerbero,GStreamer/cerbero,GStreamer/cerbero,GStreamer/cerbero,atsushieno/cerbero,nirbheek/cerbero,atsushieno/cerbero