gh-119182: Use public PyUnicodeWriter API in union_repr() #117211

Job	Run time
Check for source changes	8s
Check if generated files are up to date	5m 6s
Docs	0s
Address sanitizer	17m 19s
Ubuntu SSL tests with OpenSSL (1.1.1w)	2m 8s
Hypothesis tests on Ubuntu	4m 56s
Ubuntu SSL tests with OpenSSL (3.0.13)	1m 55s
Ubuntu SSL tests with OpenSSL (3.1.5)	1m 55s
Ubuntu SSL tests with OpenSSL (3.2.1)	2m 16s
CIFuzz (address)	19m 14s
CIFuzz (undefined)	20m 7s
CIFuzz (memory)	19m 43s
Windows / build and test (x64)	14m 12s
Windows / build (arm64)	3m 8s
Windows / build and test (x86)	15m 18s
macOS / build and test (ghcr.io/cirruslabs/macos-runner:sonoma)	5m 0s
macOS / build and test (macos-13)	8m 27s
macOS (free-threading) / build and test (ghcr.io/cirruslabs/macos-runner:sonoma)	6m 1s
Windows (free-threading) / build and test (x64)	20m 51s
Windows (free-threading) / build (arm64)	2m 33s
Windows (free-threading) / build and test (x86)	22m 50s
Thread sanitizer / Thread sanitizer	4m 35s
Thread sanitizer (free-threading) / Thread sanitizer	7m 51s
Ubuntu / build and test	6m 19s
Ubuntu (free-threading) / build and test	8m 52s
WASI / build and test	6m 36s
All required checks pass	4s
	3h 47m 24s

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