PARQUET-2495: Update merge script to python3

[emkornfield]

• Fixes differences between python2 and python3
• Unifies to only using git as gitbox is no longer necessary.
• Used to merge PARQUET-2483
• Adds options to clear existing branches used from merge, if for some reason they exist (useful while iterating on updates to the script)
• Adds the ability to pass PR number as an arg on the command line

It is possible JIRA updates still have some work here, as it errored out while trying to do those.

Make sure you have checked all steps below.

Jira

• [ x] My PR addresses the following Parquet Jira issues and references them in the PR title. For example, "PARQUET-1234: My Parquet PR"
  • https://issues.apache.org/jira/browse/PARQUET-2495
  • In case you are adding a dependency, check if the license complies with the ASF 3rd Party License Policy.

Tests

• [ x] My PR adds the following unit tests OR does not need testing for this extremely good reason: No existing unit tests and somewhat hard to test as it requires REST calls/etc

Commits

• [x ] My commits all reference Jira issues in their subject lines. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Style

• [x ] My contribution adheres to the code style guidelines and Spotless passes.
  • To apply the necessary changes, run mvn spotless:apply -Pvector-plugins

Documentation

• [ x] In case of new functionality, my PR adds documentation that describes how to use it.
  • All the public functions and the classes in the PR contain Javadoc that explain what it does