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_driver.c

@@ -163,7 +163,7 @@ int bpy_pydriver_create_dict(void)
       PyObject *arg_key, *arg_value;
       Py_ssize_t arg_pos = 0;
       while (PyDict_Next(mod_math_dict, &arg_pos, &arg_key, &arg_value)) {
-        const char *arg_str = _PyUnicode_AsString(arg_key);
+        const char *arg_str = PyUnicode_AsUTF8(arg_key);
         if (arg_str[0] && arg_str[1] != '_') {
           PyDict_SetItem(bpy_pydriver_Dict__whitelist, arg_key, Py_None);
         }
@@ -363,7 +363,7 @@ static bool bpy_driver_secure_bytecode_validate(PyObject *expr_code, PyObject *d
         fprintf(stderr,
                 "\tBPY_driver_eval() - restricted access disallows name '%s', "
                 "enable auto-execution to support\n",
-                _PyUnicode_AsString(name));
+                PyUnicode_AsUTF8(name));
         return false;
       }
     }