treewide: python-scripts: use `is None` for none-equality

Testing for `None`-ness with `==` operator is frowned upon and causes
warnings in at least "LGTM" python linter. Fix that.

Signed-off-by: Konstantin Kharlamov <Hi-Angel@yandex.ru>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>