systemc: Set verify.py shebang to python3

Change-Id: Id33658c19f6ccfe5d1552599ad7928bae99784d4
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/45980
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

src/systemc/tests/verify.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # Copyright 2018 Google, Inc.
 #