Fix failing readthedocs build, so that the documentation badge reports 'passing', not 'failing'

[artgoldberg]

de_sim's RTD badge at https://readthedocs.org/projects/de-sim/badge/?version=latest reports failing
Although its builds of docs in Circle pass, as in https://app.circleci.com/pipelines/github/KarrLab/de_sim/318/workflows/d577aa8a-a263-4593-9926-9b9f17a10b1d/jobs/394. its RTD builds are indeed failing, as one can see at https://readthedocs.org/projects/de-sim/builds/12287491/
The failure occurs on this pip install:

/home/docs/checkouts/readthedocs.org/user_builds/de-sim/envs/stable/bin/python -m pip install --use-feature 2020-resolver --exists-action=w --no-cache-dir -r docs/requirements.rtd.txt

Curiously, when logged into RTD as "karr-lab-daemon", de_sim doesn't appear in the list of repos at https://readthedocs.org/dashboard/, but it is listed as "already imported" at https://readthedocs.org/dashboard/import/?

This problem seems to be caused by a change in pip. Looking at the history of de-sim builds, they worked until Oct. 17, 2020 and started failing on Oct. 21. We see a pip bug that RTD knows about it.

In RTD issue 7605 I've asked them to put a workaround in our RTD builds until they install the fixed pip.

[artgoldberg]

Fixed.