Propagate contextvars through anyio streams #609

Job	Run time
checks / pre-commit	33s
checks / readme-snippets	15s
checks / test (3.11, locked, ubuntu-latest)	52s
checks / test (3.10, lowest-direct, ubuntu-latest)	53s
checks / test (3.10, lowest-direct, windows-latest)	2m 20s
checks / test (3.10, locked, windows-latest)	2m 20s
checks / test (3.11, locked, windows-latest)	2m 36s
checks / test (3.10, locked, ubuntu-latest)	53s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 17s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 2s
checks / test (3.12, lowest-direct, windows-latest)	4m 25s
checks / test (3.14, locked, ubuntu-latest)	2m 15s
checks / test (3.13, locked, ubuntu-latest)	45s
checks / test (3.11, lowest-direct, windows-latest)	2m 39s
checks / test (3.13, lowest-direct, ubuntu-latest)	52s
checks / test (3.12, locked, windows-latest)	3m 40s
checks / test (3.13, locked, windows-latest)	2m 12s
checks / test (3.12, locked, ubuntu-latest)	59s
checks / test (3.11, lowest-direct, ubuntu-latest)	54s
checks / test (3.13, lowest-direct, windows-latest)	2m 30s
checks / test (3.14, locked, windows-latest)	3m 17s
checks / test (3.14, lowest-direct, windows-latest)	2m 30s
all-green	5s
	40m 4s

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