Fix build failure for Docutils 0.18.0

[jfbu]

Node.findall() was added only at Docutils 0.18.1, here is extract from Docutils HISTORY

Release 0.18.1
==============

* docutils/nodes.py

  - `Node.traverse()` returns a list again to restore backwards
    compatibility. Fixes bug #431.

  - New method `Node.findall()`: like `Node.traverse()` but returns an
    iterator. Obsoletes `Node.traverse()`.

Fix #10596

Relates

• Migrate to Node.findall() from Node.traverse() #10044