Fix rich comparisons for datetime objects

[pdearnshaw]

As identified in #11, rich comparisons for cftime.datetime objects with other unknown objects do not work as cftime raises a TypeError instead of returning NotImplemented. This pull request is to fix this.

While this works in Python 3, it should be noted that Python 2 is more problematic. The unit test not_comparable_4 in test_richcmp will fail as cftime.datetime objects are treated as comparable with integer zero and so the TypeError is not raised when it is expected.

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[jswhit]

The python 2.7 Travis tests are failing. We can't merge until this is fixed.

[pdearnshaw]

This was alluded to in the description and I am still trying to work out the best way of dealing with it. At present the only way I have is to add

if sys.version_info[0] < 3:
    raise TypeError("cannot compare {0!r} and {1!r}".format(self, other))
else:
    return NotImplemented

This solves the original problem for Python 3 and allows the code to behave as before for Python 2, but in Python 2 code the original problem will still exist. As far as I can tell there is no way in Python 2 to separate objects that should be comparable to cftime.datatime objects and those that shouldn't be without being explicit (e.g. the way datetime objects are handled), or prescribing a structure on those objects that want to be comparable.

Would this be acceptable?

[jswhit]

I'm OK with this. @jhamman?

[jhamman]

I'm also fine with this but would appreciate a test to validate the expected behavior.

[jswhit]

I can merge this as soon as a test is added

[jswhit]

I'm going to go ahead and merge this, without a test for now.