Cannot pickle TZFixedOffset (and therefore datetimes with fixed offsets using C extensions)

[movermeyer]

On a system that makes use of the C extensions:

def test_pickle_with_fixed_offset():
    dt = parse_iso8601('2018-11-01 20:42:09.058000+00:00')
    pickled_bytes = pickle.dumps(dt)
    dt2 = pickle.loads(pickled_bytes)
    assert dt == dt2

raises an exception:

def test_pickle_with_fixed_offset():
        dt = parse_iso8601('2018-11-01 20:42:09.058000+00:00')
>       pickled_bytes = pickle.dumps(dt)
E       _pickle.PicklingError: Can't pickle <class 'FixedOffset_type'>: attribute lookup FixedOffset_type on builtins failed

[movermeyer]

I think that this is the C code equivalent of #161

[movermeyer]

For what it's worth, this same issue was faced and fixed in backports.datetime_fromisoformat. So it might be useful for this library too.