py/lexer: Raise SyntaxError when unicode char point out of range.

tests/unicode/unicode.py

@@ -20,3 +20,9 @@ print(enc, enc.decode() == s)
 # printing of unicode chars using repr
 # TODO we don't do this correctly
 #print(repr(s))
+
+# test invalid escape code
+try:
+    eval('"\\U00110000"')
+except SyntaxError:
+    print('SyntaxError')