Assigning to myself to review.

To add some context that hasn't come up previously, the essential idea for this macro originated in the "Py3C" extension module compatibility project, where it helps authors of Python 2 extension modules update their projects to be compatible with Python 3: https://py3c.readthedocs.io/en/latest/reference.html#comparison-helpers

As with most such pattern extractions, the key intent is to make it easier for developers to correctly implement a Python-specific protocol in C by replacing the current copy-and-paste handling of such cases with the use of C's preprocessor.