GH-38857: [Python] Fix append mode for cython 2

[jorisvandenbossche]

Rationale for this change

Small fixup of the change in #38820 to fix the build failure on cython 2 (nightly crossbow build)

• Closes: [Python] Expose append parameter from FileOutputStream::Open #38857

[jorisvandenbossche]

The question is also whether we actually need to keep this cython 2 compatibility around for longer. I think we can probably also just start requiring cython >= 3 for building pyarrow.

[jorisvandenbossche]

@github-actions crossbow submit test-conda-python-3.10-cython2

[github-actions]

Revision: 38e8a4e

Submitted crossbow builds: ursacomputing/crossbow @ actions-7f496d3260

Task	Status
test-conda-python-3.10-cython2

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 6 benchmarking runs that have been run so far on merge-commit 2bd8e06.

There was 1 benchmark result indicating a performance regression:

• Commit Run on ursa-i9-9960x at 2023-12-01 21:27:52Z
  • tpch (R) with engine=arrow, format=parquet, language=R, memory_map=False, query_id=TPCH-07, scale_factor=1

The full Conbench report has more details. It also includes information about 4 possible false positives for unstable benchmarks that are known to sometimes produce them.