pylibfdt: Fix disable version normalization

On Arch Linux based systems python setuptools does not contain
"setuptools.extern" hence it is failing with the following
error-message:
"
ModuleNotFoundError: No module named 'setuptools.extern'
"

According to a eschwartz `setuptools.extern` is not a public API and
shall not be assumed to be present in the setuptools package. He
mentions that the setuptools project anyway wants to drop this. [1]

Use the correct solution introduced by python setuptools developers to
disable normalization. [2]

[1] https://bbs.archlinux.org/viewtopic.php?id=259608
[2] https://github.com/pypa/setuptools/pull/2026


Fixes: 440098c4 ("pylibfdt: Fix version normalization warning")
Signed-off-by: Philippe Schenker <philippe.schenker@toradex.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>