Only add the YamlDriver to the chain when `symfony/yaml` is installed #426

	name: Docs

	on:
	pull_request:
	push:
	branches:
	- master

	jobs:
	build:
	runs-on: ubuntu-latest

	steps:
	- uses: actions/checkout@v2
	- name: Set up Python
	uses: actions/setup-python@v2
	with:
	python-version: 3.11
	- name: Install dependencies
	run: \|
	python -m pip install --upgrade pip
	pip install -r doc/requirements.txt --user
	- name: Build
	run: \|
	cd doc && sphinx-build -W -b html -d _build/doctrees . _build/html

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