gh-119182: Use public PyUnicodeWriter API in union_repr() (#120797)

The public PyUnicodeWriter API enables overallocation by default and
so is more efficient.

Benchmark:

python -m pyperf timeit \
    -s 't = int | float | complex | str | bytes | bytearray' \
       ' | memoryview | list | dict' \
    'str(t)'

Result:

1.29 us +- 0.02 us -> 1.00 us +- 0.02 us: 1.29x faster