> A look through `git log -p` looks like bpo-34646 is likely to be related.

New changeset e5024517811ee990b770fca0ba7058742d00e032 by Benjamin Peterson in branch 'master':
closes bpo-34646: Remove PyAPI_* macros from declarations. (GH-9218)
https://github.com/python/cpython/commit/e5024517811ee990b770fca0ba7058742d00e032

Extract of this change:

 
 #undef PyRun_String
-PyAPI_FUNC(PyObject *)
+PyObject *
 PyRun_String(const char *str, int s, PyObject *g, PyObject *l)
 {
     return PyRun_StringFlags(str, s, g, l, NULL);
 }
 

On Windows, this change removed dllexport from PyRun_String(). My PR 14142 adds it back, to pythonrun.h.