Build: resolve error with Python 3.14

Include local version of _PyArg_CheckPositional which is private
for CPython as of Python 3.14.

Ref !147783

source/blender/python/mathutils/mathutils_geometry.cc

@@ -29,6 +29,7 @@
 #include "BLI_utildefines.h"
 
 #include "../generic/py_capi_utils.hh"
+#include "../generic/python_compat.hh" /* IWYU pragma: keep. */
 #include "../generic/python_utildefines.hh"
 
 /* ---------------------------------INTERSECTION FUNCTIONS-------------------- */