WIP: Fix CPython tests

[mre]

See #8

[mre]

@konstin thanks to the fix for PyO3/pyo3#177 I was able to add our own JSONDecodeError to hyperjson (yay!).
This fixes a few unit tests and that makes me very happy.

One issue that keeps popping up is, that the msg field is missing from our JSONDecodeError:

    def test_unexpected_data(self):
        test_cases = [
            ('[,', 'Expecting value', 1),
            ('{"spam":[}', 'Expecting value', 9),
            ('[42:', "Expecting ',' delimiter", 3),
            ('[42 "spam"', "Expecting ',' delimiter", 4),
            ('[42,]', 'Expecting value', 4),
            ('{"spam":[42}', "Expecting ',' delimiter", 11),
            ('["]', 'Unterminated string starting at', 1),
            ('["spam":', "Expecting ',' delimiter", 7),
            ('["spam",]', 'Expecting value', 8),
            ('{:', 'Expecting property name enclosed in double quotes', 1),
            ('{,', 'Expecting property name enclosed in double quotes', 1),
            ('{42', 'Expecting property name enclosed in double quotes', 1),
            ('[{]', 'Expecting property name enclosed in double quotes', 2),
            ('{"spam",', "Expecting ':' delimiter", 7),
            ('{"spam"}', "Expecting ':' delimiter", 7),
            ('[{"spam"]', "Expecting ':' delimiter", 8),
            ('{"spam":}', 'Expecting value', 8),
            ('[{"spam":]', 'Expecting value', 9),
            ('{"spam":42 "ham"', "Expecting ',' delimiter", 11),
            ('[{"spam":42]', "Expecting ',' delimiter", 11),
            ('{"spam":42,}', 'Expecting property name enclosed in double quotes', 11),
        ]
        for data, msg, idx in test_cases:
            with self.assertRaises(self.JSONDecodeError) as cm:
                self.loads(data)
            err = cm.exception
>           self.assertEqual(err.msg, msg)
E           AttributeError: 'JSONDecodeError' object has no attribute 'msg'

I think, fixing that would make quite a few more tests pass.
The question is, what's the most idiomatic way to add the msg attribute here?
I could go and copy the output of the py_exception! macro to the code, but I wonder if there's a better way.

One thing that would be really fancy is supporting custom derives on structs to make them Python exceptions. Something like

#[derive(PyException)]
struct JSONDecodeError {
  msg: String,
}

Have you considered this? I'm a bit reluctant to open an issue on pyo3, because... well... it's not really an issue. 😉

[konstin]

The easiest fix is to just add a msg field to the python object by name. Otherwise with PyO3/pyo3#291 everything is ready to implement a custom (Which should make a Good First Issue)