This is a minimal (and currently incomplete) example on how to develop, build, package, and test a project that requires python 2.7 support.
This specifically attempts to adopt modern tools from python 3.10, and bolt on 2.7 support.
If you are in this situation, I do not envy you.
See CONTRIBUTING on all the details for local test, dev, and build
https://github.com/python/mypy/blob/4687cec37a2a28e477e0fcf7eb95d2701bea55eb/docs/source/python2.rst