Cython compile time conditional statements are deprecated

[snowman2]

cython/cython#4310

[snowman2]

cython/cython#4310 (comment)

The current situation is something like "Cython core developers don't like this feature and would like to replace it with a better alternative". It's a statement of intent rather than immediate action. The main consequence right now is that we aren't interesting in proposals extend the current conditional complication feature. ... We'll make sure that suitable warnings appear in Cython well before anything removes, and that the alternatives are well-documented. Until then I'd wait (especially with respect to existing code).