Issue #8784: Set tarfile default encoding to 'utf-8' on Windows.

Note: file system encoding cannot be None anymore (since r81190, issue #8610).
diff --git a/Lib/tarfile.py b/Lib/tarfile.py
index 31967dd..4839eb1 100644
--- a/Lib/tarfile.py
+++ b/Lib/tarfile.py
@@ -159,9 +159,10 @@
 #---------------------------------------------------------
 # initialization
 #---------------------------------------------------------
-ENCODING = sys.getfilesystemencoding()
-if ENCODING is None:
-    ENCODING = "ascii"
+if os.name in ("nt", "ce"):
+    ENCODING = "utf-8"
+else:
+    ENCODING = sys.getfilesystemencoding()
 
 #---------------------------------------------------------
 # Some useful functions