Propagate contextvars through anyio streams #668

Job	Run time
checks / pre-commit	46s
checks / readme-snippets	14s
checks / test (3.10, locked, windows-latest)	2m 13s
checks / test (3.11, lowest-direct, ubuntu-latest)	56s
checks / test (3.10, lowest-direct, windows-latest)	2m 33s
checks / test (3.10, locked, ubuntu-latest)	50s
checks / test (3.10, lowest-direct, ubuntu-latest)	59s
checks / test (3.12, locked, ubuntu-latest)	56s
checks / test (3.12, lowest-direct, windows-latest)	3m 31s
checks / test (3.11, lowest-direct, windows-latest)	2m 43s
checks / test (3.11, locked, windows-latest)	2m 35s
checks / test (3.11, locked, ubuntu-latest)	53s
checks / test (3.13, lowest-direct, ubuntu-latest)	53s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 6s
checks / test (3.14, locked, ubuntu-latest)	2m 14s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 21s
checks / test (3.13, lowest-direct, windows-latest)	2m 54s
checks / test (3.13, locked, ubuntu-latest)	49s
checks / test (3.13, locked, windows-latest)	2m 36s
checks / test (3.14, locked, windows-latest)	4m 37s
checks / test (3.14, lowest-direct, windows-latest)	2m 28s
checks / test (3.12, locked, windows-latest)	3m 10s
all-green	5s
	41m 22s

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