Use correct kwarg in handle_exception() for Flask (#1300)

* Use correct kwarg in handle_exception() for Flask

The handle_exception() method is registered as a receiver of the
got_request_exception signal. According to both the [documentation][1]
and [source code][2] of Flask the got_request_exception signal passes
the exception as `exception` rather than a `exc_info`.

This is likely to have gone unnoticed since captureException() calls
sys.exc_info() in the absence of an exception.

On Python 3 we can use `__traceback__` to explicitly construct the
`exc_info`.

[1]: http://flask.pocoo.org/docs/1.0/api/#flask.got_request_exception
[2]: https://github.com/pallets/flask/blob/1.0.2/flask/app.py#L1732

* doc: Add changelog

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