Refactor add-bugzilla-versions into a Bash script that runs a Python package

Summary: I created a python package at tools/python-kde-release, included a Bash script helper to run commands from the Python package (including transparently creating or updating a virtual environment), and created an add_bugzilla_versions.sh Bash script in the root of this repository that uses the helper Bash script to run the corresponding Python method.

Test Plan: I manually tested add_bugzilla_version.sh with --dry.

Differential Revision: https://phabricator.kde.org/D24134