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GH-41527: [CI][Dev] Remove unncessary requirements for six (#43087)



### Rationale for this change

The `six` requirement is a compatibility library for Python 2 and 3. Since Arrow no longer supports Python 2, we need to remove `six` requirement from merge script ([Python compatibility](https://arrow.apache.org/docs/python/install.html#python-compatibility)).

### What changes are included in this PR?

- Removing `six` requirement
- Updating `merge_arrow_pr.py`
  - `six.string_types` to `str`
  - `six_binary_type` to `bytes`

### Are these changes tested?

Tested by CI.

### Are there any user-facing changes?

Maybe not. It is for developers.

* GitHub Issue: #41527

Authored-by: Hyunseok Seo <hsseo0501@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>