Can plpython3u open files on the file system?
It is quite possible.  But the OS user running the PostgreSQL server (usually 'postgres') might not have permission to traverse the directory tree containing the file even if it has permissions to the file itself.  Or might be in a chroot jail and so can't even see the directory.  Or it could be blocked by SELinux policy.  This is system's level stuff, nothing specific to PostgreSQL or even databases generally.