Support webpdf format #59
Description
nbconvert v6.0 introduced a new webpdf format that generates a PDF via HTML rather than via LaTeX.
https://nbconvert.readthedocs.io/en/latest/usage.html#convert-webpdf
This may need some work on dependencies for CI, since it requires a headless Chromium browser. The documentation suggests that simply installing pyppeteer might be enough.