perf scripts python: Add Python 3 support to SchedGui.py

Fix a single syntax error in SchedGui.py to support both Python 2 and
Python 3. This should have no functional change.

Signed-off-by: Jeremy Cline <jeremy@jcline.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Herton Krzesinski <herton@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/0100016341a72d26-75729663-fe55-4309-8c9b-302e065ed2f1-000000@email.amazonses.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>