Bump actions/checkout from 3 to 4 #667

Job	Run time
Build docs & run doctests	33s
Verify dev env (ubuntu-latest)	18s
Check types using pyright	26s
Verify dev env (windows-latest)	57s
Tests & Mypy on 3.7	1m 36s
Tests & Mypy on 3.8	1m 23s
Tests & Mypy on 3.9	1m 18s
Tests & Mypy on 3.10	1m 40s
Tests & Mypy on 3.11	1m 33s
Tests & Mypy on 3.12	2m 2s
Tests & Mypy on pypy-3.8	1m 3s
Tests & Mypy on pypy-3.9	1m 2s
Tests & Mypy on pypy-3.10	1m 6s
Combine & check coverage.	15s
required-checks-pass	3s
	15m 15s

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