Propagate contextvars through anyio streams #496

Job	Run time
checks / readme-snippets	11s
checks / pre-commit	35s
checks / test (3.10, lowest-direct, windows-latest)	2m 38s
checks / test (3.11, lowest-direct, windows-latest)	2m 33s
checks / test (3.12, locked, windows-latest)	3m 21s
checks / test (3.11, locked, windows-latest)	2m 55s
checks / test (3.12, lowest-direct, windows-latest)	3m 48s
checks / test (3.10, locked, ubuntu-latest)	46s
checks / test (3.10, lowest-direct, ubuntu-latest)	44s
checks / test (3.12, locked, ubuntu-latest)	52s
checks / test (3.13, lowest-direct, ubuntu-latest)	55s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 7s
checks / test (3.10, locked, windows-latest)	2m 18s
checks / test (3.14, locked, ubuntu-latest)	2m 14s
checks / test (3.13, lowest-direct, windows-latest)	2m 51s
checks / test (3.13, locked, windows-latest)	2m 35s
checks / test (3.11, lowest-direct, ubuntu-latest)	44s
checks / test (3.11, locked, ubuntu-latest)	45s
checks / test (3.14, lowest-direct, windows-latest)	2m 10s
checks / test (3.13, locked, ubuntu-latest)	46s
checks / test (3.12, lowest-direct, ubuntu-latest)	50s
checks / test (3.14, locked, windows-latest)	3m 4s
all-green	4s
	38m 46s

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