use python3 in hack

hack/verify-flags-underscore.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Copyright 2015 The Kubernetes Authors.
 #
@@ -14,8 +14,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from __future__ import print_function
-
 import argparse
 import os
 import re
@@ -107,7 +105,7 @@ def check_underscore_in_flags(rootdir, files):
         print("Are you certain this flag should not have been declared with an - instead?")
         l = list(new_excluded_flags)
         l.sort()
-        print("%s" % "\n".join(l))
+        print(("%s" % "\n".join(l)))
         sys.exit(1)
 
 def main():