Please adapt to new version of sphinxcontrib-bibtex

[tillea]

Hi,
there is a bug report against the Debian packaged version of cogent3 which fails with:

Extension error:
You must configure the bibtex_bibfiles setting

I did some research about this error and found hints like this where the solution is to pin sphinxcontrib-bibtex<2.0.0 to requirements.txt. While this might work for local users it is no option for the Debian packaging since sphinx is moving on and we have currently version 2.4.1 to which we need to adapt the code. I will try to consult some people who know sphinx better than I and may be I can come up with a patch but I just want to let you know here.
Kind regards, Andreas.

[tillea]

BTW, I've got a hint that bibtex might not be used at all and removing all references to bibtex solved the issue in dead.
Kind regards, Andreas.