Change some uses of cStringIO.StringIO to io.StringIO.

This is undoubtedly insufficient and in some cases just as broken as before.

Lib/tarfile.py

@@ -2459,10 +2459,7 @@ class TarFileCompat:
     def write(self, filename, arcname=None, compress_type=None):
         self.tarfile.add(filename, arcname)
     def writestr(self, zinfo, bytes):
-        try:
-            from cStringIO import StringIO
-        except ImportError:
-            from StringIO import StringIO
+        from io import StringIO
         import calendar
         zinfo.name = zinfo.filename
         zinfo.size = zinfo.file_size