Bump actions/checkout from 3 to 4 (#60) #99

Job	Run time
Type-check package	28s
Build docs & run doctests	27s
Run linters	48s
Verify dev env (ubuntu-latest)	28s
Tests on 3.8	1m 21s
Verify dev env (windows-latest)	1m 5s
Tests on 3.9	1m 45s
Verify dev env (macos-latest)	1m 43s
Tests on 3.10	1m 30s
Tests on 3.11	1m 18s
Tests on 3.12	2m 0s
Tests on pypy-3.8	1m 38s
Tests on pypy-3.9	1m 24s
Combine & check coverage.	11s
required-checks-pass	2s
	16m 8s

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