graph-depends.py: support python3

This patch is the result of 2to3.

In addition, universal_newlines=True is added to the Popen calls. In
python3, this makes sure that the output is decoded so that we get a
string instead of a buffer object.

Cc: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>