Fix nbconvert pdf export

[martinRenou]

Fix #403

When exporting to pdf, the only mimetype that makes sense is the image/png one, so we need it around.

[github-actions]

👈 Launch a binder notebook on branch martinRenou/ipympl/fix_nbconvert_pdf_export

[tovrstra]

Thanks for the quick fix!! (I've also tested it locally and the fix works.)

[martinRenou]

Thanks for trying :)

I see another issue on the HTML export with nbconvert. I'm trying to push a fix for that too and should release 0.8.3.