make lib2to3 parse async generators everywhere (GH-6588)

(cherry picked from commit 149addd496)

Co-authored-by: Zsolt Dollenstein <zsol.zsol@gmail.com>

Lib/lib2to3/tests/test_parser.py

@@ -200,20 +200,27 @@ def test_async_var(self):
         self.validate("""await = 1""")
         self.validate("""def async(): pass""")
 
-    def test_async_with(self):
+    def test_async_for(self):
         self.validate("""async def foo():
                              async for a in b: pass""")
 
-        self.invalid_syntax("""def foo():
-                                   async for a in b: pass""")
-
-    def test_async_for(self):
+    def test_async_with(self):
         self.validate("""async def foo():
                              async with a: pass""")
 
         self.invalid_syntax("""def foo():
                                    async with a: pass""")
 
+    def test_async_generator(self):
+        self.validate(
+            """async def foo():
+                   return (i * 2 async for i in arange(42))"""
+        )
+        self.validate(
+            """def foo():
+                   return (i * 2 async for i in arange(42))"""
+        )
+
 
 class TestRaiseChanges(GrammarTest):
     def test_2x_style_1(self):