gh-144220: Copy metadata before files in shutil.copytree()

[vstinner]

shutil.copytree() now copies directory metadata before copying files. It prevents an error when setting an extended attribute (bcachefs.casefold) on an non-empty directory.

• Issue: shutil.copytree fails on bcachefs case insensitive directories #144220

[vstinner]

This PR changes st_mtime of the destination directory:

$ mkdir src/ 
$ echo hello >src/hello.txt
$ ./python -c 'import shutil; shutil.copytree("src", "dst")'
$ ./python
>>> import os; src=os.stat("src"); dst=os.stat("dst")
>>> dst.st_mtime_ns == src.st_mtime_ns
False

Without this change, the st_mtime_ns comparison returns True.

[vstinner]

This change doesn't work if the source directory has the UF_IMMUTABLE flag: copytree() fails with PermissionError on copying files to the destination.

Example on FreeBSD 15:

$ mkdir src/
$ echo hello > src/hello.txt
$ ./python -c 'import os, stat; os.chflags("src", stat.UF_IMMUTABLE)'
$ ./python -c 'import shutil; shutil.copytree("src", "dst")'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
    import shutil; shutil.copytree("src", "dst")
                   ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^
  File "/home/vstinner/python/main/Lib/shutil.py", line 656, in copytree
    return _copytree(entries=entries, src=src, dst=dst, symlinks=symlinks,
                     ignore=ignore, copy_function=copy_function,
                     ignore_dangling_symlinks=ignore_dangling_symlinks,
                     dirs_exist_ok=dirs_exist_ok)
  File "/home/vstinner/python/main/Lib/shutil.py", line 610, in _copytree
    raise Error(errors)
shutil.Error: [('src/hello.txt', 'dst/hello.txt', "[Errno 1] Operation not permitted: 'dst/hello.txt'")]

[vstinner]

My PR changes st_mtime and breaks on an UF_IMMUTABLE directory. I prefer to abandon my PR. See the issue for a discussion on how to solve this problem.