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

[miss-islington]

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 79efbb7)

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

https://bugs.python.org/issue24538

[miss-islington]

@obilaniu and @giampaolo: Status check is done, and it's a failure ❌ .

[miss-islington]

@obilaniu and @giampaolo: Status check is done, and it's a failure ❌ .

[tonybaloney]

Looks like a flaky test

======================================================================
ERROR: test_drain_raises (test.test_asyncio.test_streams.StreamTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "D:\a\1\s\lib\test\test_asyncio\test_streams.py", line 820, in test_drain_raises
    self.loop.run_until_complete(client(*addr))
  File "D:\a\1\s\lib\asyncio\base_events.py", line 584, in run_until_complete
    return future.result()
  File "D:\a\1\s\lib\test\test_asyncio\test_streams.py", line 810, in client
    await writer.drain()
  File "D:\a\1\s\lib\asyncio\streams.py", line 351, in drain
    await fut
  File "D:\a\1\s\lib\asyncio\selector_events.py", line 860, in write
    n = self._sock.send(data)
ConnectionAbortedError: [WinError 10053] An established connection was aborted by the software in your host machine

[bedevere-bot]

@giampaolo: Please replace # with GH- in the commit message next time. Thanks!