Propagate contextvars through anyio streams #737

Job	Run time
checks / pre-commit	34s
checks / readme-snippets	15s
checks / test (3.11, lowest-direct, ubuntu-latest)	53s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 2s
checks / test (3.10, lowest-direct, windows-latest)	2m 22s
checks / test (3.10, locked, ubuntu-latest)	49s
checks / test (3.11, locked, ubuntu-latest)	50s
checks / test (3.11, locked, windows-latest)	2m 18s
checks / test (3.12, locked, ubuntu-latest)	55s
checks / test (3.12, lowest-direct, windows-latest)	3m 46s
checks / test (3.13, lowest-direct, ubuntu-latest)	55s
checks / test (3.14, lowest-direct, windows-latest)	2m 50s
checks / test (3.12, locked, windows-latest)	3m 18s
checks / test (3.13, locked, ubuntu-latest)	47s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 18s
checks / test (3.13, lowest-direct, windows-latest)	2m 40s
checks / test (3.11, lowest-direct, windows-latest)	2m 31s
checks / test (3.13, locked, windows-latest)	2m 33s
checks / test (3.14, locked, ubuntu-latest)	2m 19s
checks / test (3.10, locked, windows-latest)	2m 13s
checks / test (3.14, locked, windows-latest)	3m 51s
checks / test (3.10, lowest-direct, ubuntu-latest)	51s
all-green	5s
	39m 55s

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