Bump actions/download-artifact from 3 to 4 #101

Job	Run time
Run linters	29s
Type-check package	18s
Tests on 3.8	1m 15s
Tests on 3.9	1m 5s
Tests on 3.10	1m 5s
Tests on 3.11	1m 0s
Tests on 3.12	1m 18s
Build docs & run doctests	26s
Tests on pypy-3.8	1m 12s
Tests on pypy-3.9	1m 13s
Verify dev env (ubuntu-latest)	20s
Verify dev env (windows-latest)	46s
Verify dev env (macos-latest)	37s
Combine & check coverage.	8s
required-checks-pass	4s
	11m 16s

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