[$] HPy: a better C API for Python?

The HPy project has beenaround for more than a year now; it is meant to provide an alternate C APIfor Python that allows extensions to the language to run, and run well, in moreenvironments. It first came to our attention in a reportof a talk at the 2020Python Language Summit (LWN coverage), but it goes back to some discussions that wereheld at EuroPython 2019. Thereare a number of ways that the existing C API holds back innovation forPython, but there are also some hugely important extensions (e.g. NumPy) that use it; any change to the APIneeds to take those into account.