arrow

GH-40439: [Python] Fix flake8 failures in python/benchmarks/parquet.py (#40440)

### Rationale for this change

Failures:

    /arrow/python/benchmarks/parquet.py:18:1: F401 'shutil' imported but unused
    /arrow/python/benchmarks/parquet.py:19:1: F401 'tempfile' imported but unused
    /arrow/python/benchmarks/parquet.py:23:1: F401 'pandas as pd' imported but unused

### What changes are included in this PR?

* Remove unused imports.
* Add python/benchmarks/ to lint targets.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* GitHub Issue: #40439

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>