Bump actions/download-artifact from 3 to 4 #159

Job	Run time
📦 Build the source distribution	13s
Ensure cogified files are up-to-date	12s
Verify dev env on ubuntu-latest	25s
Verify dev env on windows-latest	1m 18s
Build docs & run doctests	24s
Tests on 3.8	40s
Mypy on 3.8	14s
Tests on 3.9	27s
Mypy on 3.9	14s
Tests on 3.10	20s
Mypy on 3.10	14s
Tests on 3.11	19s
Mypy on 3.11	13s
Tests on 3.12	22s
Mypy on 3.12	19s
Combine & check coverage	7s
required-checks-pass	2s
	6m 3s

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