Use python3 for installed executable python scripts.

Fedora is a Python3-only distribution:
https://fedoraproject.org/wiki/FinalizingFedoraSwitchtoPython3

This fixes build failures where builders may strictly enforce only
python3 during a transitional phase.

Author: Carlos O'Donell <carlos@redhat.com>

diff --git a/benchtests/scripts/compare_bench.py b/benchtests/scripts/compare_bench.py
index ea25f778c09bba9d..b53beb3c6e32c3cf 100755
--- a/benchtests/scripts/compare_bench.py
+++ b/benchtests/scripts/compare_bench.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/python3
 # Copyright (C) 2015-2018 Free Software Foundation, Inc.
 # This file is part of the GNU C Library.
 #
diff --git a/benchtests/scripts/import_bench.py b/benchtests/scripts/import_bench.py
index 602b3f954d4801a6..76bf1528a5418748 100644
--- a/benchtests/scripts/import_bench.py
+++ b/benchtests/scripts/import_bench.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/python3
 # Copyright (C) 2015-2018 Free Software Foundation, Inc.
 # This file is part of the GNU C Library.
 #
diff --git a/benchtests/scripts/validate_benchout.py b/benchtests/scripts/validate_benchout.py
index 6147f05bec3a4844..9a5c7947ee4ed7e9 100755
--- a/benchtests/scripts/validate_benchout.py
+++ b/benchtests/scripts/validate_benchout.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/python3
 # Copyright (C) 2014-2018 Free Software Foundation, Inc.
 # This file is part of the GNU C Library.
 #