g.extension: use copytree from shutil for py >= 3.8

[ninsbl]

Here is a suggestion to address #2016
Not sure if it needs to be backported all the way to 7.8 as people with such an old installation, probably do not have more recent Python anyway. So maybe 8.0 is a sufficient milestone.

An alternative implementation would be choose the function later in the code, like:

if sys.version_info.major == 3 and sys.version_info.minor < 8:
        from distutils.dir_util import copy_tree

And then here:

grass/scripts/g.extension/g.extension.py

Line 1582 in 82fb7b8

copy_tree(actual_file, os.path.join(target_dir, file_name))

if sys.version_info.major == 3 and sys.version_info.minor < 8:
    copy_tree(actual_file, os.path.join(target_dir, file_name))
else:
    shutil.copytree(actual_file, os.path.join(target_dir, file_name), dirs_exist_ok=True)

[petrasovaa]

Didn't test, but looks good! I would say 8 is sufficient.

[ninsbl]

Backported to 8.0 in 20bee01
and 8.2 in c780325