stringio doesn't have an encoding

Lib/_pyio.py

@@ -1834,3 +1834,7 @@ class StringIO(TextIOWrapper):
         # TextIOWrapper tells the encoding in its repr. In StringIO,
         # that's a implementation detail.
         return object.__repr__(self)
+
+    @property
+    def encoding(self):
+        return None

Lib/test/test_memoryio.py

@@ -451,7 +451,7 @@ class PyStringIOTest(MemoryTestMixin, MemorySeekTestMixin, unittest.TestCase):
 
         # These are just dummy values but we nevertheless check them for fear
         # of unexpected breakage.
-        self.assertEqual(memio.encoding, "utf-8")
+        self.assertTrue(memio.encoding is None)
         self.assertEqual(memio.errors, "strict")
         self.assertEqual(memio.line_buffering, False)
 

Modules/_stringio.c

@@ -665,7 +665,7 @@ stringio_encoding(StringIOObject *self, void *context)
 {
     CHECK_INITIALIZED(self);
     CHECK_CLOSED(self);
-    return PyUnicode_FromString("utf-8");
+    Py_RETURN_NONE;
 }
 
 static PyObject *