Basic support for PEP 414 without docs or tests.
diff --git a/Python/ast.c b/Python/ast.c
index c565642..0f93098 100644
--- a/Python/ast.c
+++ b/Python/ast.c
@@ -3796,6 +3796,9 @@
                 quote = *++s;
                 *bytesmode = 1;
             }
+            else if (quote == 'u' || quote == 'U') {
+                quote = *++s;
+            }
             else if (quote == 'r' || quote == 'R') {
                 quote = *++s;
                 rawmode = 1;