Use PEP 508-compliant marker #808

Job	Run time
Check types using pyright	27s
Build docs & run doctests	31s
Tests & Mypy on 3.7	41s
Verify dev env (ubuntu-latest)	22s
Tests & Mypy on 3.8	1m 3s
Verify dev env (windows-latest)	1m 4s
Tests & Mypy on 3.9	1m 7s
Tests & Mypy on 3.10	1m 21s
Tests & Mypy on 3.11	1m 3s
Tests & Mypy on 3.12	1m 25s
Tests & Mypy on pypy-3.8	1m 4s
Tests & Mypy on pypy-3.9	56s
Tests & Mypy on pypy-3.10	1m 2s
Combine & check coverage.	20s
required-checks-pass	2s
	12m 28s

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