Pin pygments version in RTD build. (#4267)
This fixes our RTD failures, which were caused by RTD installing an older version of pygments:
```
jupyterlab-pygments 0.1.1 requires pygments<3,>=2.4.1, but you'll have pygments 2.3.1 which is incompatible.
nbconvert 6.0.1 requires pygments>=2.4.1, but you'll have pygments 2.3.1 which is incompatible.
```
