bpo-40334: Add type to the assignment rule in the grammar file (GH-19963)

diff --git a/Grammar/python.gram b/Grammar/python.gram
index 0ce6ab4..3f16768 100644
--- a/Grammar/python.gram
+++ b/Grammar/python.gram
@@ -82,7 +82,7 @@
     | &'while' while_stmt
 
 # NOTE: annotated_rhs may start with 'yield'; yield_expr must start with 'yield'
-assignment:
+assignment[stmt_ty]:
     | a=NAME ':' b=expression c=['=' d=annotated_rhs { d }] {
         CHECK_VERSION(
             6,