Update all development Yarn dependencies (2020-08-28)

This is your weekly update of all development Yarn dependencies. Please take a good look at what changed and the test results before merging this pull request.

What changed?

✳️ prettier (2.0.5 → 2.1.1, minor) · Repo · Changelog · Release · Diff

✳️ mocha (8.1.1 → 8.1.2, patch) · Repo · Changelog · Release · Diff

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