Bump flatted from 3.2.9 to 3.4.2 in /components/dash-core-components #1031

Job	Run time
Detect Relevant Path Changes	4s
Build Dash Package	3m 10s
Typing Tests	2m 38s
Table Unit/Lint Tests (Python ${{ matrix.python-version }})	0s
Table Server Tests (Group ${{ matrix.test-group }})	0s
Run Background & Async Callback Tests (Python ${{ matrix.python-version }})	0s
	5m 52s

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