ovs-tcpundump: Fix incompatibilities with python3

ovs-tcpundump: Fix incompatibilities with python3

Added parenthesis after print and use "as" instead of "," in except.

This commit fixes also a couple of flake8 warnings:

    utilities/ovs-tcpundump:23:1: E302 expected 2 blank lines, found 1
    utilities/ovs-tcpundump:35:1: E305 expected 2 blank lines after class or
    function definition, found 1

Tested on Python 2.7.15 and Python 3.6.5

Signed-off-by: Timothy Redaelli <tredaelli@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>