Test

sketch out sync codecs + threadpool #6269

Job	Run time
doctests	18s
py=3.13-upstream	24s
py=3.11-min_deps	17s
os=ubuntu-latest, py=3.13, deps=optional	20s
os=ubuntu-latest, py=3.12, deps=optional	21s
os=ubuntu-latest, py=3.11, deps=minimal	16s
os=ubuntu-latest, py=3.13, deps=minimal	16s
os=ubuntu-latest, py=3.11, deps=optional	21s
os=macos-latest, py=3.13, deps=optional	19s
os=macos-latest, py=3.11, deps=optional	24s
os=ubuntu-latest, py=3.12, deps=minimal	19s
os=windows-latest, py=3.11, deps=optional	23s
os=windows-latest, py=3.13, deps=optional	21s
Test complete	3s
	4m 22s

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