Fix spack info bug for Python 3 (#4391)

lib/spack/spack/cmd/info.py

@@ -25,7 +25,7 @@
 from __future__ import print_function
 
 import textwrap
-import itertools
+from six.moves import zip_longest
 from llnl.util.tty.colify import *
 import spack
 import spack.fetch_strategy as fs
@@ -119,7 +119,7 @@ def lines(self):
                     v.description,
                     width=self.column_widths[2]
                 )
-                for t in itertools.izip_longest(
+                for t in zip_longest(
                         name, allowed, description, fillvalue=''
                 ):
                     yield "    " + self.fmt % t