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 a215ebc9bd92ba73..f174161ed3147e03 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) 2015-2020 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 08681aafdc200f64..2264da00370f9f02 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) 2015-2020 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 715218aff804baa0..6b4877a67b86b791 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) 2014-2020 Free Software Foundation, Inc.
 # This file is part of the GNU C Library.
 #