Add `pathlib.Path.copytree()` method, which recursively copies one
directory to another.
This differs from `shutil.copytree()` in the following respects:
1. Our method has a *follow_symlinks* argument, whereas shutil's has a
*symlinks* argument with an inverted meaning.
2. Our method lacks something like a *copy_function* argument. It always
uses `Path.copy()` to copy files.
3. Our method lacks something like a *ignore_dangling_symlinks* argument.
Instead, users can filter out danging symlinks with *ignore*, or
ignore exceptions with *on_error*
4. Our *ignore* argument is a callable that accepts a single path object,
whereas shutil's accepts a path and a list of child filenames.
5. We add an *on_error* argument, which is a callable that accepts
an `OSError` instance. (`Path.walk()` also accepts such a callable).
Co-authored-by: Nice Zombies <nineteendo19d0@gmail.com>
35e998f560