Support 'mbcs' as a 'built-in' encoding, so the C API can use it without

defering to the encodings package.
As described in [ 763111 ] mbcs encoding should skip encodings package

Objects/unicodeobject.c

@@ -531,6 +531,10 @@ PyObject *PyUnicode_Decode(const char *s,
         return PyUnicode_DecodeUTF8(s, size, errors);
     else if (strcmp(encoding, "latin-1") == 0)
         return PyUnicode_DecodeLatin1(s, size, errors);
+#if defined(MS_WINDOWS) && defined(HAVE_USABLE_WCHAR_T)
+    else if (strcmp(encoding, "mbcs") == 0)
+        return PyUnicode_DecodeMBCS(s, size, errors);
+#endif
     else if (strcmp(encoding, "ascii") == 0)
         return PyUnicode_DecodeASCII(s, size, errors);
 
@@ -591,6 +595,10 @@ PyObject *PyUnicode_AsEncodedString(PyObject *unicode,
 	    return PyUnicode_AsUTF8String(unicode);
 	else if (strcmp(encoding, "latin-1") == 0)
 	    return PyUnicode_AsLatin1String(unicode);
+#if defined(MS_WINDOWS) && defined(HAVE_USABLE_WCHAR_T)
+	else if (strcmp(encoding, "mbcs") == 0)
+	    return PyUnicode_AsMBCSString(unicode);
+#endif
 	else if (strcmp(encoding, "ascii") == 0)
 	    return PyUnicode_AsASCIIString(unicode);
     }
@@ -2621,6 +2629,17 @@ PyObject *PyUnicode_EncodeMBCS(const Py_UNICODE *p,
     return repr;
 }
 
+PyObject *PyUnicode_AsMBCSString(PyObject *unicode)
+{
+    if (!PyUnicode_Check(unicode)) {
+        PyErr_BadArgument();
+        return NULL;
+    }
+    return PyUnicode_EncodeMBCS(PyUnicode_AS_UNICODE(unicode),
+				PyUnicode_GET_SIZE(unicode),
+				NULL);
+}
+
 #endif /* MS_WINDOWS */
 
 /* --- Character Mapping Codec -------------------------------------------- */