[FIX] Fixes for nbconvert > 6

[AakashGfude]

This PR solves the following:

• not reading article.tpxl for jinja extend of latex.tpl
• not able to read html.tpl using jinja2 Remove early version requirements for dask #328

The reason behind these till now has been identified as the major release of nbconvert which is 6.0.x

fixes QuantEcon/lecture-python#329

[mmcky]

thanks @AakashGfude are there still issues with the tplx?

Can you work through the travis build to get his passing CI

1. pin sphinxcontrib-bibtex==1.0 should fix this.
2. Raise an issue for sphinxcontrib-bibtex>=2 compatibility

[AakashGfude]

thanks @AakashGfude are there still issues with the tplx?

Yes, there have been quite drastic changes in nbconvert 6 with regards to templates. I Will have to do edits/refactor on the template files. Doing that at the moment

1. pin sphinxcontrib-bibtex==1.0 should fix this.
2. Raise an issue for sphinxcontrib-bibtex>=2 compatibility

Thanks @mmcky . Will do this.