List of python implementations of specreduce-like functionality

This feels like the right place to keep track of existing implementations of specreduce functionality. Though I'm pretty sure we have a googledoc somewhere that also records this.

* https://github.com/AWehrhahn/PyReduce
* https://github.com/GeminiDRSoftware/DRAGONS
* https://github.com/pypeit/PypeIt

Review the issue and its three linked Python implementations first. The payload names no repository file, test, or entry point; done means establishing where this list belongs and recording the relevant implementations there without relying on the separate Google Doc.

Written by the indexing model from the issue text.