feat: Add Release 21 example and CI (!1)
This was inspired by https://atlas-talk.web.cern.ch/t/running-recast-locally-not-working/677/4.
* Add Release 21 AnalysisBase + Python 2 virtual environment example.
* Add links to AnalysisBase releases in README.
* Add Makefile for local tests.
* Add CI
Edited by Matthew Feickert