Drop support for the "ur" string prefix

Parser/tokenizer.c

--- a/Parser/tokenizer.c
+++ b/Parser/tokenizer.c
@@ -1412,7 +1412,7 @@
     /* Identifier (most frequent token!) */
     nonascii = 0;
     if (is_potential_identifier_start(c)) {
-        /* Process b"", r"", u"", br"", rb"" and ur"" */
+        /* Process b"", r"", u"", br"" and rb"" */
         int saw_b = 0, saw_r = 0, saw_u = 0;
         while (1) {
             if (!(saw_b || saw_u) && (c == 'b' || c == 'B'))
@@ -1421,7 +1421,8 @@
                want to support it in arbitrary order like byte literals. */
             else if (!(saw_b || saw_u || saw_r) && (c == 'u' || c == 'U'))
                 saw_u = 1;
-            else if (!saw_r && (c == 'r' || c == 'R'))
+            /* ur"" and ru"" are not supported */
+            else if (!(saw_r || saw_u) && (c == 'r' || c == 'R'))
                 saw_r = 1;
             else
                 break;