Update .readthedocs.yml - add build.os #741

	name: Build documentation on PR

	on: pull_request

	jobs:
	build-and-deploy:
	runs-on: ubuntu-latest
	steps:
	- uses: actions/checkout@v2
	with:
	submodules: "recursive"
	token: ${{ secrets.ALLTHEACTIONS }}
	- name: Set up Python 3.7
	uses: actions/setup-python@v1
	with:
	python-version: "3.7"
	- name: Install dependencies
	run: \|
	python -m pip install --upgrade pip
	python -m pip install mkdocs
	python -m pip install -r requirements.txt
	# TODO: use this to check for broken links in docs/ ?
	#- name: Check for dead links in Markdown
	# uses: gaurav-nelson/[email protected]
	- name: Build site
	run: mkdocs build

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