GH-49295: [Python] Remove "mimalloc" from mandatory_backends

[kou]

Rationale for this change

mimalloc is enabled by default but users can disable it. For example, Debian package disables it.

If it's disabled, PyArrow tests are failed.

What changes are included in this PR?

Remove "mimalloc" from mandatory_backends in our test.

Are these changes tested?

Yes.

Are there any user-facing changes?

No.

• GitHub Issue: [Python] test_memory.py fails with -DARROW_MIMALLOC=OFF #49295

[github-actions]

⚠️ GitHub issue #49295 has been automatically assigned in GitHub to PR creator.

[Copilot]

Pull request overview

This PR updates PyArrow’s memory-pool tests to avoid assuming mimalloc is always available, aligning the test expectations with builds where mimalloc is disabled (e.g., some downstream distro packages).

Changes:

• Remove "mimalloc" from mandatory_backends so tests only require the "system" allocator backend.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[raulcd]

As per the conversation on the issue, LGTM

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 3 benchmarking runs that have been run so far on merge-commit ddc4229.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 59 possible false positives for unstable benchmarks that are known to sometimes produce them.