Build script does not download new versions of jupyterlab css files

[twanvl]

In hatch_build.py, a bunch of css files are downloaded. The urls include a version number (jupyterlab_css_version = "4.0.2"). But the script checks if the files already exists, and does not download them in that case. If previously an older version of the nbconvert was built, which downloaded older versions of the css files, then the css files are not updated.

I noticed this because the Debian python3-nbconvert package actually includes an old version of template/lab/style.css, which leads to weird styling issues. There might be more such subtly incorrect builds of nbconvert out there.

There should be a check to see that the existing files match jupyterlab_css_version.

[cjwatson]

Note that the proposed change here wouldn't have helped with the Debian bug in question anyway - Debian package builds can't download from the internet, so the Debian nbconvert package carries a patch to disable downloading CSS and embeds the files in question instead. (I'm updating them now.)