Propagate contextvars through anyio streams #579

Job	Run time
checks / pre-commit	20s
checks / test (3.13, lowest-direct, windows-latest)	2m 30s
checks / test (3.12, locked, ubuntu-latest)	55s
checks / test (3.11, lowest-direct, windows-latest)	2m 34s
checks / test (3.14, locked, ubuntu-latest)	2m 18s
checks / test (3.10, locked, windows-latest)	2m 22s
checks / test (3.10, lowest-direct, ubuntu-latest)	48s
checks / test (3.10, locked, ubuntu-latest)	49s
checks / test (3.11, lowest-direct, ubuntu-latest)	58s
checks / test (3.13, locked, windows-latest)	2m 21s
checks / test (3.11, locked, windows-latest)	2m 26s
checks / test (3.12, locked, windows-latest)	3m 11s
checks / test (3.13, lowest-direct, ubuntu-latest)	41s
checks / test (3.11, locked, ubuntu-latest)	43s
checks / test (3.12, lowest-direct, ubuntu-latest)	51s
checks / test (3.12, lowest-direct, windows-latest)	4m 6s
checks / test (3.13, locked, ubuntu-latest)	51s
checks / readme-snippets	14s
checks / test (3.14, lowest-direct, windows-latest)	2m 25s
checks / test (3.10, lowest-direct, windows-latest)	2m 28s
checks / test (3.14, locked, windows-latest)	3m 15s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 16s
all-green	2s
	38m 24s

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