simplebench/bench_block_job: use correct shebang line with python3

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210116214705.822267-22-vsementsov@virtuozzo.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

scripts/simplebench/bench_block_job.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # Benchmark block jobs
 #