scripts: switch explicitly to Python 3

Some distributions are about to switch to Python 3 support only.
This means that /usr/bin/python, which is Python 2, is not available
anymore. Hence, switch scripts to use Python 3 explicitly.
Signed-off-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>

scripts/bloat-o-meter

-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # Copyright 2004 Matt Mackall <mpm@selenic.com>
 #

scripts/diffconfig

-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: GPL-2.0
 #
 # diffconfig - a tool to compare .config files.