format: Update merge script

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](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=uwe) / @xhochy
**Assignee**: [Uwe Korn](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=uwe) / @xhochy
#### PRs and other links:
- [GitHub Pull Request #158](https://github.com/apache/parquet-format/pull/158)

_{**Note**: *This issue was originally created as [PARQUET-1779](https://issues.apache.org/jira/browse/PARQUET-1779). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Start by reviewing the existing merge script and merge_parquet.py, then compare them with the Arrow project's merge_script referenced in the issue. Check pull request #158 for the work already undertaken and its expected compatibility behavior. Done means the merge script works with Python 3 and matches the intended Arrow-based implementation.

Written by the indexing model from the issue text.