fix: Handle `decimal.Decimal` instances in flattening #4040

Job	Run time
External Tests	2m 23s
Test on 3.7 (tests) / ubuntu-latest / SQLAlchemy: 2.*	1m 16s
Test on 3.8 (tests) / ubuntu-latest / SQLAlchemy: 2.*	1m 10s
Test on 3.9 (tests) / ubuntu-latest / SQLAlchemy: 2.*	1m 17s
Test on 3.10 (tests) / ubuntu-latest / SQLAlchemy: 2.*	1m 5s
Test on 3.11 (tests) / ubuntu-latest / SQLAlchemy: 2.*	1m 23s
Test on 3.7 (tests) / macos-latest / SQLAlchemy: 2.*	2m 20s
Test on 3.8 (tests) / macos-latest / SQLAlchemy: 2.*	2m 17s
Test on 3.9 (tests) / macos-latest / SQLAlchemy: 2.*	2m 11s
Test on 3.10 (tests) / macos-latest / SQLAlchemy: 2.*	2m 29s
Test on 3.11 (tests) / macos-latest / SQLAlchemy: 2.*	2m 52s
Test on 3.7 (tests) / windows-latest / SQLAlchemy: 2.*	2m 48s
Test on 3.8 (tests) / windows-latest / SQLAlchemy: 2.*	2m 40s
Test on 3.9 (tests) / windows-latest / SQLAlchemy: 2.*	2m 41s
Test on 3.10 (tests) / windows-latest / SQLAlchemy: 2.*	3m 29s
Test on 3.11 (tests) / windows-latest / SQLAlchemy: 2.*	2m 32s
Test on 3.11 (tests) / ubuntu-latest / SQLAlchemy: 1.*	1m 33s
Test on 3.10 (doctest) / ubuntu-latest / SQLAlchemy: 2.*	40s
Test on 3.8 (mypy) / ubuntu-latest / SQLAlchemy: 2.*	56s
Coverage	37s
	38m 39s

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