Fix T85573: Building with Python 3.10a5 fails

Replace deprecated _PyUnicode_AsString{AndSize} usage.

T83626 still needs to be resolved before 3.10 is usable.

source/blender/python/intern/bpy_operator.c

@@ -58,7 +58,7 @@
 
 static wmOperatorType *ot_lookup_from_py_string(PyObject *value, const char *py_fn_id)
 {
-  const char *opname = _PyUnicode_AsString(value);
+  const char *opname = PyUnicode_AsUTF8(value);
   if (opname == NULL) {
     PyErr_Format(PyExc_TypeError, "%s() expects a string argument", py_fn_id);
     return NULL;