This is done to avoid troubles with %lang files listed as duplicated.
1. It gets rid of a warning
2. It fixes a problem described in:
http://lists.rpm.org/pipermail/rpm-list/2020-November/002041.html
This is a backwards incompatible change,
packages that rename or remove the installed files after %pyproject_install
might no longer be compatible with %pyproject_save_files.