[build] Make IWYU scipts compatible with Python 3
[build] Make IWYU scipts compatible with Python 3

This patch makes minor adjustments to iwyu.py to make it
compatible with Python 3.

Change-Id: I979e96f66261146571ffc45f8222ad0fc15d1073
Reviewed-on: http://gerrit.cloudera.org:8080/15031
Tested-by: Kudu Jenkins
Reviewed-by: Alexey Serbin <864f6b82bd94e8dda64141d4274519ee66e0102e@cloudera.com>

helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu