Issue #13639: Accept unicode filenames in tarfile.open(mode="w|gz").

Passing a unicode filename to tarfile.open() along with mode "w|gz" failed
with a UnicodeError because the filename was not encoded properly before being
written to the gzipped stream in the FNAME extra field.
diff --git a/Lib/tarfile.py b/Lib/tarfile.py
index ed5bcf2..bd73965 100644
--- a/Lib/tarfile.py
+++ b/Lib/tarfile.py
@@ -454,6 +454,8 @@
                                             0)
         timestamp = struct.pack("<L", long(time.time()))
         self.__write("\037\213\010\010%s\002\377" % timestamp)
+        if type(self.name) is unicode:
+            self.name = self.name.encode("iso-8859-1", "replace")
         if self.name.endswith(".gz"):
             self.name = self.name[:-3]
         self.__write(self.name + NUL)