Don't use datetime timestamp() as it's not in Python 2
The `timestamp()` method for `datetime.datetime` instances was
only added in Python 3. It does not exist in Python 2. So we
can't safely use it here.
I believe this should do the right thing, taking advantage of a
couple of existing utility functions in koji.util.
Signed-off-by: Adam Williamson <awilliam@redhat.com>