[Dev] merge_arrow_pr.py depends on six

[asfimport]

This is causing Source Release and Merge Script CI to fail because it used to get six transitively, but a dependency updated and now we don't get  it.

Reporter: David Li / @lidavidm

Related issues:

• [CI] Merge script test fails due to missing dependency (duplicates)

_{Note: This issue was originally created as ARROW-12775. Please see the migration documentation for further details.}