git_remote_helpers: allow building with Python 3

Change inline Python to call "print" as a function not a statement.

This is harmless because Python 2 will see the parentheses as redundant
grouping but they are necessary to run this code with Python 3.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>