bpo-39390: Update shutil.copytree doc - Add versionchanged 3.8 notice for ignore callable

[mbarkhau]

The directory being copied.

In [21]: !tree
.
├── derp.txt
├── herp.txt
└── subdir
    └── subdireentry.txt

1 directory, 3 files

An ignore function for debugging.

In [23]: def _ignore(*args):
    ...:     print("ignore args:", args)
    ...:     return []

Python 3.7

In [25]: shutil.copytree(".", "/tmp/copytree37", ignore=_ignore)
ignore args: ('.', ['derp.txt', 'herp.txt', 'subdir'])
ignore args: ('./subdir', ['subdireentry.txt'])
Out[25]: '/tmp/copytree37'
In [26]: sys.version
Out[26]: '3.7.5 (default, Nov 20 2019, 09:21:52) \n[GCC 9.2.1 20191008]'

Python 3.8

In [32]: shutil.copytree(".", "/tmp/copytree38", ignore=_ignore)
ignore args: ('.', {'herp.txt', 'subdir', 'derp.txt'})
ignore args: (<DirEntry 'subdir'>, {'subdireentry.txt'})
Out[32]: '/tmp/copytree38'
In [33]: sys.version
Out[33]: '3.8.1 | packaged by conda-forge | (default, Jan  5 2020, 20:58:18) \n[GCC 7.3.0]'

https://bugs.python.org/issue39390

[mbarkhau]

The "skip news" label can probably be added.

[mbarkhau]

This PR may be obsolete. The alternative way to deal with this issue would be to revert to the old behaviour, which may be the preferable way.

[mbarkhau]

Closing in favour of #18122