Use dataclass to reduce boilerplate in event definitions

[cburgdorf]

What was wrong?

This is an alternative to #63 relying on the new @dataclass decorator that was added with Python 3.7. There's a backport for Python 3.6 available as well: https://pypi.org/project/dataclasses/

How was it fixed?

Use dataclasses in examples

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[cburgdorf]

It fails for the py36-examples job because I haven't used the Python 3.6 fallback here but we could do that. But on the other hand, if it's easy to build our own DataEvent with a metaclass that might be even better.

[cburgdorf]

Ok, I went ahead and added the dataclasses backport for the py36 job. I read up a bit on https://docs.python.org/3/library/dataclasses.html and I'm leaning towards just going with that as dataclasses come with a bunch of other features and so this may just end up as the more flexible solution. Thoughts?