python3: silence DeprecationWarnings in python3-config

Our patched python3-config uses distutils, but that will emit a
DeprecationWarning when imported.  This isn't useful when using the
output of python3-config to find include paths.

This isn't a huge problem typically as the warning goes to stderr, but
some build systems read both stdout and stderr.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>