Update PyUnicode_DecodeUTF8 from RFC 2279 to RFC 3629.

1) #8271: when a byte sequence is invalid, only the start byte and all the
   valid continuation bytes are now replaced by U+FFFD, instead of replacing
   the number of bytes specified by the start byte.
   See http://www.unicode.org/versions/Unicode5.2.0/ch03.pdf (pages 94-95);
2) 5- and 6-bytes-long UTF-8 sequences are now considered invalid (no changes
   in behavior);
3) Change the error messages "unexpected code byte" to "invalid start byte"
   and "invalid data" to "invalid continuation byte";
4) Add an extensive set of tests in test_unicode;
5) Fix test_codeccallbacks because it was failing after this change.

Lib/test/test_codeccallbacks.py

@@ -153,28 +153,30 @@ class CodecCallbackTest(unittest.TestCase):
             sout += bytes("\\U%08x" % sys.maxunicode, "ascii")
         self.assertEqual(sin.encode("iso-8859-15", "backslashreplace"), sout)
 
-    def test_decoderelaxedutf8(self):
-        # This is the test for a decoding callback handler,
-        # that relaxes the UTF-8 minimal encoding restriction.
-        # A null byte that is encoded as "\xc0\x80" will be
-        # decoded as a null byte. All other illegal sequences
-        # will be handled strictly.
+    def test_decoding_callbacks(self):
+        # This is a test for a decoding callback handler
+        # that allows the decoding of the invalid sequence
+        # "\xc0\x80" and returns "\x00" instead of raising an error.
+        # All other illegal sequences will be handled strictly.
         def relaxedutf8(exc):
             if not isinstance(exc, UnicodeDecodeError):
                 raise TypeError("don't know how to handle %r" % exc)
-            if exc.object[exc.start:exc.end].startswith(b"\xc0\x80"):
+            if exc.object[exc.start:exc.start+2] == b"\xc0\x80":
                 return ("\x00", exc.start+2) # retry after two bytes
             else:
                 raise exc
 
-        codecs.register_error(
-            "test.relaxedutf8", relaxedutf8)
+        codecs.register_error("test.relaxedutf8", relaxedutf8)
 
+        # all the "\xc0\x80" will be decoded to "\x00"
         sin = b"a\x00b\xc0\x80c\xc3\xbc\xc0\x80\xc0\x80"
         sout = "a\x00b\x00c\xfc\x00\x00"
         self.assertEqual(sin.decode("utf-8", "test.relaxedutf8"), sout)
+
+        # "\xc0\x81" is not valid and a UnicodeDecodeError will be raised
         sin = b"\xc0\x80\xc0\x81"
-        self.assertRaises(UnicodeError, sin.decode, "utf-8", "test.relaxedutf8")
+        self.assertRaises(UnicodeDecodeError, sin.decode,
+                          "utf-8", "test.relaxedutf8")
 
     def test_charmapencode(self):
         # For charmap encodings the replacement string will be