Fix wrong way of adding Error information in shutil.copytree.

Lib/shutil.py

@@ -236,7 +236,7 @@ def copytree(src, dst, symlinks=False, ignore=None, copy_function=copy2,
             # Copying file access times may fail on Windows
             pass
         else:
-            errors.extend((src, dst, str(why)))
+            errors.append((src, dst, str(why)))
     if errors:
         raise Error(errors)