> get_platform() will be difficult to reuse, for bootstrapping reasons

ISTM that if you can't determine the value at compile time, then it doesn't matter to compilation enough to need to appear in the tag.

As far as matching PEP 425 for the sake of matching it goes, I'd rather keep using search path tricks than have ".cp35-cp35m-win_amd64.pyd" appear on every single one of my extension modules. Removing the "_d" suffix is very likely more disruptive than it's worth, especially since untagged pyds are still supported but the debug tag is still necessary. 'm' is always the case in Windows and is benign for a correct extension anyway, and AFAICT 'u' is totally unused.