Bump actions/checkout from 3 to 4 #98

Job	Run time
Build docs & run doctests	27s
Run linters	40s
Tests on 3.8	1m 28s
Tests on 3.9	1m 23s
Verify dev env (ubuntu-latest)	27s
Tests on 3.10	1m 42s
Verify dev env (windows-latest)	1m 22s
Tests on 3.11	1m 16s
Verify dev env (macos-latest)	41s
Tests on 3.12	1m 45s
Tests on pypy-3.8	1m 23s
Tests on pypy-3.9	1m 23s
Type-check package	25s
Combine & check coverage.	15s
required-checks-pass	3s
	14m 40s

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