bpo-24538: Fix bug in shutil involving the copying of xattrs to read-only files....

bpo-24538: Fix bug in shutil involving the copying of xattrs to read-only files. (PR-13212) (#13234)

Extended attributes can only be set on user-writeable files, but shutil previously
first chmod()ed the destination file to the source's permissions and then tried to
copy xattrs. This will cause failures if attempting to copy read-only files with
xattrs, as occurs with Git clones on Lustre FS.
(cherry picked from commit 79efbb719383386051c72f2ee932eeca8e033e6b)

Co-authored-by: Olexa Bilaniuk <obilaniu@users.noreply.github.com>