gh-113188: Fix shutil.copymode() on Windows (GH-113189)

gh-113188: Fix shutil.copymode() on Windows (GH-113189)

Previously it worked differently if dst is a symbolic link:
it modified the permission bits of dst itself rather than the file
it points to if follow_symlinks is true or src is not a symbolic link,
and did nothing if follow_symlinks is false and src is a symbolic link.

Also document similar changes in shutil.copystat().