Get the parser working with "async yield from".

Author: ZeroIntensity

Grammar/python.gram

@@ -127,7 +127,7 @@ simple_stmt[stmt_ty] (memo):
     | &'raise' raise_stmt
     | &'pass' pass_stmt
     | &'del' del_stmt
-    | &'yield' yield_stmt
+    | &('yield' | 'async') yield_stmt
     | &'assert' assert_stmt
     | &'break' break_stmt
     | &'continue' continue_stmt
@@ -724,6 +724,7 @@ if_expression[expr_ty]:
 yield_expr[expr_ty]:
     | 'yield' 'from' a=expression { _PyAST_YieldFrom(a, EXTRA) }
     | 'yield' a=[star_expressions] { _PyAST_Yield(a, EXTRA) }
+    | 'async' 'yield' 'from' a=expression { _PyAST_AsyncYieldFrom(a, EXTRA) }
 
 star_expressions[expr_ty]:
     | a=star_expression b=(',' c=star_expression { c })+ [','] {

Include/internal/pycore_ast.h

@@ -73,6 +73,7 @@ module Python
          | Await(expr value)
          | Yield(expr? value)
          | YieldFrom(expr value)
+         | AsyncYieldFrom(expr value)
          -- need sequences for compare to distinguish between
          -- x < 4 < 3 and (x < 4) < 3
          | Compare(expr left, cmpop* ops, expr* comparators)