Adds DataTriples which act just like DataPairs

[janpf]

This adds a new DataPoint type consisting of three sentences instead of two. This could/should likely be parameterizable, but this is the easiest way to add this.

[alanakbik]

Thanks for the PR! Just some minor mypy problems, concerning implicit optionals. I've added some suggestions as comments to fix the issues.

[alanakbik]

@janpf thanks a lot for adding this! Will you also add some prepared corpora for the SuperGLEBler datasets in future PRs?

[janpf]

@janpf thanks a lot for adding this! Will you also add some prepared corpora for the SuperGLEBler datasets in future PRs?

Thanks for the quick merge, @alanakbik :)
We are thinking about it. Currently, we provide the datasets via the repo, which was the fastest way to deliver the files. I think we'll make sure version 0.13 works nicely, and then we'll see :)

Is there an ETA for when the two PRs might land on pypi? :)

[alanakbik]

I'm hoping to find enough time to prepare a new release once the semester ends (in two weeks).