There is also switchlang¹ which isn't quite the same thing, but provides some of the functionality of PEP-622 and pampy. I believe it is notable for including a nice descriptive README, and also having a small/simple implementation.

I personally prefer the pampy internals, but quite like the context manager usage from switchlang. I don't even know which bikeshed I want to paint, let alone the colour.

1. https://github.com/mikeckennedy/python-switch