Test mt940.models.FixedOffset fails on Python 3.7

[dotlambda]

______________________ [doctest] mt940.models.FixedOffset ______________________
017 Fixed time offset based on the Python docs
018     Source: https://docs.python.org/2/library/datetime.html#tzinfo-objects
019
020     >>> offset = FixedOffset(60)
021     >>> offset.utcoffset(None)
Expected:
    datetime.timedelta(0, 3600)
Got:
    datetime.timedelta(seconds=3600)

/build/source/mt940/models.py:21: DocTestFailure

[wolph]

Yes, it seems that Python 3 has changed the repr() for a timedelta. It's an improvement, but inconvenient for doctests :P