Since the need to copy file ownership is common, I think there could be space for a new copy3() function which copies ownership + extended attributes (where possible). In detail:

- on Windows this can be achieved by using CopyFileEx, which would also provide better overall performances than the current readinto() implementation. This was the original approach used in BPO-33671, discarded after Eryk pointed out that ACLs should not be copied.

- on macOS we can use fcopyfile() (which is already exposed privately) + COPYFILE_ALL, see http://www.manpagez.com/man/3/fcopyfile/

- on all other UNIX we can use the current shutil.copystat() (which already copies xattrs) + os.chown(). I am not entirely sure this would achieve a "full ACLs copy" though, or what would be needed exactly to achieve that.