Sphinx build is broken

```
PY_TREES_DISABLE_COLORS=1 sphinx-build -E -b html doc doc/html
Running Sphinx v1.8.5

Extension error:
Could not import extension sphinx.builders.latex (exception: cannot import name 'contextfunction' from 'jinja2' (/mnt/mervin/workspaces/devel/py_trees/src/py_trees/.venv/lib/python3.8/site-packages/jinja2/__init__.py))
make: *** [Makefile:21: docs] Error 2
```