Add Roundup Issue Tracker to the Ticketing section. (#2855)

The Roundup Issue Tracker is celebrating its 20th anniversary. It has been the tracking software for bugs.python.org since 2006. Please consider adding this software under active development to your list.

README.md

@@ -1817,6 +1817,7 @@ _Related: [Task management & To-do lists](#task-management--to-do-lists), [Softw
 - [OTOBO](https://otobo.de/en/) - Flexible web-based ticketing system used for Customer Service, Help Desk, IT Service Management. ([Demo](https://otobo.de/en/open-source-ticketing-system/#demos), [Source Code](https://github.com/RotherOSS/otobo)) `GPL-3.0` `Perl`
 - [Pachno](https://pach.no/) - Bring your team together to design, build and deliver your project with a tool that works with you and your team, and adapts when you need to. ([Source Code](https://github.com/pachno/pachno)) `MPL-2.0` `PHP`
 - [Request Tracker](https://www.bestpractical.com/rt/) - An enterprise-grade issue tracking system. ([Source Code](https://github.com/bestpractical/rt)) `GPL-2.0` `Perl`
+- [Roundup Issue Tracker](https://www.roundup-tracker.org/) - A simple-to-use and -install issue-tracking system with command-line, web, REST, XML-RPC, and e-mail interfaces. Designed with flexibility in mind - not just another bug tracker. ([Source Code](https://www.roundup-tracker.org/code.html)) `MIT/ZPL-2.0` `Python`
 - [Sentry On-Premise](https://github.com/getsentry/onpremise) - A powerful error tracking platform with wide language support and a robust API. ([Source Code](https://github.com/getsentry/sentry)) `BSD-3-Clause` `Python/Django`
 - [SIT](https://sit.fyi/) - SCM-agnostic, file-based, offline-first, immutable issue tracker. ([Source Code](https://github.com/sit-fyi/sit)) `MIT` `Apache-2.0` `Rust`
 - [Trudesk](https://trudesk.io/) - Trudesk is an open-source help desk/ticketing solution. ([Source Code](https://github.com/polonel/trudesk)) `Apache-2.0` `Nodejs`