Remove sphinx and nbsphinx from requirements.txt file

[yuvipanda]

These aren't actually required for the package to function, and since setup.py reads requirements.txt, these packages get installed each time someone installs datascience package. This becomes a problem for Jupyterlite (similar to #522) as nbsphinx and sphinx pull in a lot of dependencies, many which can't be installed on jupyterlite

[yuvipanda]

@ericvd-ucb this is also required to get the datascience package working on jupyterlite