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: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>