[Decoration] Exporting of rendered plots fails on windows

[github-actions]

closes #369

[rihi]

Note, that the same issue theoretically exists for export_ascii aswell. However as far as I could see, graph-easy only reads input from files, so passing it via stdin is not possible.

[rihi]

I think we could use in-memory "files" for the exporting tests. Then the tests don't have to interact with the filesystem. See https://docs.python.org/3/library/io.html#binary-i-o
Oh, sorry just realised that export_plot always writes to a file. Perhaps it would make sense to change the api of export_plot to allow specifying an output handle?

[blattm]

Thank you very much for your contribution!