e2e tests with pytest-mpl

[PiotrZakrzewski]

After making some changes to pigeon (see my other PR on updating dnaplotlob) I wanted to test it.
I did it by running some example scripts in this repo, this process can be automated with pytest-mpl.

I can contribute a change introducing some simple pytest-mpl based e2e suite (along with github workflow to run it).
What do you think?

[rkrishnasanka]

That's a good idea, I was thinking that we should do some e2e testing but I wasn't sure how we could test the image outputs, this library should do the trick.