format: Update merge script

[asfimport]

The current merge script is Python 3 incompatible, copy over the merge_script from the Arrow project which is a development that initially started from merge_parquet.py

Reporter: Uwe Korn / @xhochy
Assignee: Uwe Korn / @xhochy

PRs and other links:

• GitHub Pull Request #158

_{Note: This issue was originally created as PARQUET-1779. Please see the migration documentation for further details.}