timedelta microseconds are stored improperly

[vsevolod-fedorov]

When we store timedelta python type values to database, it's microseconds are stored improperly:
0:00:00.000005 becomes 0:00:00.500000.
Code sample to reproduce this:

#!/usr/bin/env python

from __future__ import print_function

import os
from datetime import timedelta

try:
    import psycopg2cffi as psycopg2
except ImportError:
    import psycopg2


connection = psycopg2.connect("user=postgres host=shelter password=%s" % os.environ['PGPASSWORD'])
cursor = connection.cursor()
print(cursor.mogrify('select %s;', [timedelta(microseconds=5)]))

With psycopg2 it prints

b"select '0 days 0.000005 seconds'::interval;"

With psycopg2cffi:

b"select '0 days 0.5 seconds'::interval;"