Updating to docutils 0.18 breaks the build

[mvdpanne]

We currently do not pin the version of the docutils package (as part of requirements.txt). As a result, on each build we will install the latest version of the package.

On 26-10-2021, version 0.18 was released. As a result, the build of #816 started failing (see here). The specific error is the following:

Exception occurred:
  File "/home/travis/virtualenv/python3.6.7/lib/python3.6/site-packages/sphinx/util/nodes.py", line 94, in apply_source_workaround
    for classifier in reversed(node.parent.traverse(nodes.classifier)):
TypeError: 'generator' object is not reversible

For now, I've pinned the version of docutils to 0.17.1 as part of #818. Eventually we should figure out what the root cause of this is, and fix it.