Fix LaTeX exporting '?' for non-ascii title

[imtsuki]

I have the same issue with #786 . When I tried to export a PDF file from a notebook that has a non-ascii title (which is the notebook's file name), the output is just '????':

I discovered that it was not a XeLaTeX problem; I use ctex package to support non-ascii rendering (You can see them in the date under the title), but the title is still '???'. I tried to directly export .tex source file instead, and it turned out that nbconvert itself produces '\title{????}' for title while other non-ascii characters in the document body is fine.

In nbconvert/templates/latex/base.tplx, I found this line confusing:

\title{((( nb_title | ascii_only | escape_latex )))}

I have no idea what ascii_only is for, and deleting this is supposed to make sense.

[t-makaro]

LGTM! I think ascii_only is a remnant from before we were using XeLaTeX. I will merge once tests pass. The failures look unrelated.

Also, thanks for the tip on ctex. Are you just using a custom template to add \usepackage{ctex}?

[imtsuki]

Yeah, I added that line to article.tplx for my own use, and it only supports CJK characters (as far as I know).

[meeseeksmachine]

This pull request has been mentioned on Jupyter Community Forum. There might be relevant details there:

https://discourse.jupyter.org/t/nbconvert-5-6-0-release/1867/1