pyerr: improve debug & display impls

[davidhewitt]

This PR aims to improve the ergonomics of PyErr's debug output.

Now, instead of PyErr { type: 0x... } the repr of type and value are printed (using Python::with_gil to acquire the GIL if needed). We already acquire the GIL in the Display impl so I think we have acceptable precedent to do this. It also makes the output much more useful for PyO3 users.

As a simplification, I also removed the special impl Debug and impl Display for PyException. They now just use the underlying Python object's .repr() and .str(), just like PyAny. I think there's a strong consistency argument for doing this.

Before:

PyErr debug         = PyErr { type: 0x7ff6f7b50420 }
PyErr display       = ValueError: bar
PyException debug   = ValueError
PyException display = ValueError: bar
PyAny debug         = ValueError('bar')
PyAny display       = bar

After:

PyErr debug         = PyErr { type: <class 'ValueError'>, value: ValueError('bar'), traceback: None }
PyErr display       = ValueError: bar
PyException debug   = ValueError('bar')
PyException display = bar
PyAny debug         = ValueError('bar')
PyAny display       = bar

[kngwyu]

Awesome 💯

[kngwyu]

Looks this assertion fails at Python 3.6. Maybe you can just weaken the assertion.

[davidhewitt]

👍 modified slightly to check the individual fields and use #[cfg(Py_3_7)] on the bit that's different