I found that (g)vim.exe with +python3 on Win32 exports a symbol Py3Init_vim,
but the symbol doesn't need to be exported because only PyImport_AppendInittab()
uses Py3Init_vim.
I think that "PyMODINIT_FUNC" can be replaced by "static PyObject *".
Here is a reference for PyImport_AppendInittab().
http://docs.python.org/3/c-api/import.html#PyImport_AppendInittab
Attached patch fixes this problem.
Best regards,
Ken Takata