explicit use of fixture decorator

[miguelgrinberg]

This was one of the last suggestions in #1782, to use explicit @pytest.fixture and @pytest_asyncio.fixture decorators. I inadvertently left this change out of that PR, so here it is.

[pquentin]

Thanks! LGTM.

[github-actions]

The backport to 8.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-8.x 8.x
# Navigate to the new working tree
cd .worktrees/backport-8.x
# Create a new branch
git switch --create backport-1783-to-8.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 ff406e9a09ee0dc39dc75693c86f80de43540b1b
# Push it to GitHub
git push --set-upstream origin backport-1783-to-8.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-8.x

Then, create a pull request where the base branch is 8.x and the compare/head branch is backport-1783-to-8.x.