[3.11] gh-111942: Remove an extra incref in textiowrapper_change_encoding

There's an issue with the 3.11 backport commit e2421a3
The chane for the main branch was:

```diff
+    Py_INCREF(errors);
 ...
     Py_SETREF(self->encoding, encoding);
-    Py_SETREF(self->errors, Py_NewRef(errors));
+    Py_SETREF(self->errors, errors);
```

but in 3.11 this became:

```diff
+    Py_INCREF(errors);
...
     Py_INCREF(errors);
     Py_SETREF(self->encoding, encoding);
     Py_SETREF(self->errors, errors);
```
i.e. there's an extra incref, but it looks -- at least to Git -- like the change
that removes `Py_NewRef` was already applied.

This was not caught because `test_io` refleaks tests were ineffective on 3.11
(see #126478 (comment)).

Remove the extraneous incref.

Author: encukou

Modules/_io/textio.c

@@ -1313,7 +1313,6 @@ textiowrapper_change_encoding(textio *self, PyObject *encoding,
     }
     Py_DECREF(codec_info);
 
-    Py_INCREF(errors);
     Py_SETREF(self->encoding, encoding);
     Py_SETREF(self->errors, errors);