bpf: Add Python 3 support to selftests scripts for bpf

Adjust tcp_client.py and tcp_server.py to work with Python 3 by using
the print function, marking string literals as bytes, and using the
newer exception syntax. This should be functionally equivalent and
supports Python 3+.

Signed-off-by: Jeremy Cline <jcline@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>