[xattr] Always use UTF-8

On Windows and other systems, other encodings would break when trying to encode non-ASCII characters.
Simply use UTF-8, like every sane system.

youtube_dl/postprocessor/xattrpp.py

@@ -99,7 +99,7 @@ def write_xattr(path, key, value):
                     if infoname == "upload_date":
                         value = hyphenate_date(value)
 
-                    byte_value = value.encode(preferredencoding())
+                    byte_value = value.encode('utf-8')
                     write_xattr(filename, xattrname, byte_value)
 
             return True, info