docs: Expand docs on when and why allow_threads is necessary #361

	# This runs as a separate job because it needs to run on the `pull_request_target` event
	# in order to access the CODECOV_TOKEN secret.
	#
	# This is safe because this doesn't run arbitrary code from PRs.

	name: Set Codecov PR base
	on:
	# See safety note / doc at the top of this file.
	pull_request_target:

	jobs:
	coverage-pr-base:
	runs-on: ubuntu-latest
	steps:
	- uses: actions/checkout@v4
	- uses: actions/setup-python@v5
	with:
	python-version: '3.12'
	- name: Set PR base on codecov
	run: \|
	# fetch the merge commit between the PR base and head
	git fetch -u --progress --depth=1 origin "+$BASE_REF:$BASE_REF" "+$MERGE_REF:$MERGE_REF"
	while [ -z "$(git merge-base "$BASE_REF" "$MERGE_REF")" ]; do
	git fetch -u -q --deepen="10" origin "$BASE_REF" "$MERGE_REF";
	done

	MERGE_BASE=$(git merge-base "$BASE_REF" "$MERGE_REF")
	echo "Merge base: $MERGE_BASE"

	# inform codecov about the merge base
	pip install codecov-cli
	codecovcli pr-base-picking \
	--base-sha $MERGE_BASE \
	--pr ${{ github.event.number }} \
	--slug PyO3/pyo3 \
	--token ${{ secrets.CODECOV_TOKEN }} \
	--service github
	env:
	# Don't put these in bash, because we don't want the expansion to
	# risk code execution
	BASE_REF: "refs/heads/${{ github.event.pull_request.base.ref }}"
	MERGE_REF: "refs/pull/${{ github.event.pull_request.number }}/merge"

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