Fixup old, invalid import.

Lib/cookielib.py

@@ -57,7 +57,7 @@ def _warn_unhandled_exception():
     # There are a few catch-all except: statements in this module, for
     # catching input that's bad in unexpected ways.  Warn if any
     # exceptions are caught there.
-    import warnings, traceback, StringIO
+    import io, warnings, traceback
     f = io.StringIO()
     traceback.print_exc(None, f)
     msg = f.getvalue()