Propagate contextvars through anyio streams #505

Job	Run time
checks / pre-commit	37s
checks / readme-snippets	14s
checks / test (3.10, lowest-direct, windows-latest)	2m 25s
checks / test (3.12, lowest-direct, ubuntu-latest)	49s
checks / test (3.11, lowest-direct, ubuntu-latest)	39s
checks / test (3.10, lowest-direct, ubuntu-latest)	42s
checks / test (3.12, lowest-direct, windows-latest)	4m 10s
checks / test (3.10, locked, ubuntu-latest)	50s
checks / test (3.13, lowest-direct, windows-latest)	2m 35s
checks / test (3.11, lowest-direct, windows-latest)	2m 40s
checks / test (3.11, locked, windows-latest)	2m 31s
checks / test (3.10, locked, windows-latest)	2m 18s
checks / test (3.14, lowest-direct, windows-latest)	2m 27s
checks / test (3.11, locked, ubuntu-latest)	47s
checks / test (3.13, locked, ubuntu-latest)	47s
checks / test (3.14, locked, ubuntu-latest)	2m 18s
checks / test (3.13, lowest-direct, ubuntu-latest)	44s
checks / test (3.14, locked, windows-latest)	3m 37s
checks / test (3.12, locked, ubuntu-latest)	52s
checks / test (3.13, locked, windows-latest)	2m 21s
checks / test (3.12, locked, windows-latest)	3m 55s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 7s
all-green	4s
	39m 29s

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