[3.8] bpo-39390 shutil: fix argument types for ignore callback (GH-18122)

diff --git a/Lib/shutil.py b/Lib/shutil.py
index 755ce39..cde7b86 100644
--- a/Lib/shutil.py
+++ b/Lib/shutil.py
@@ -442,7 +442,7 @@
 def _copytree(entries, src, dst, symlinks, ignore, copy_function,
               ignore_dangling_symlinks, dirs_exist_ok=False):
     if ignore is not None:
-        ignored_names = ignore(src, {x.name for x in entries})
+        ignored_names = ignore(os.fspath(src), [x.name for x in entries])
     else:
         ignored_names = set()