py39: fix mypyc complaint

I was trying to build wheels for Python 3.9 as part of python#9536, but ran
into this issue. You'll notice a couple hundred lines up msullivan
points out that mypyc can't handle conditional method definition, so
that's not an option here.

mypy/fastparse.py

@@ -1257,11 +1257,13 @@ def visit_Slice(self, n: ast3.Slice) -> SliceExpr:
 
     # ExtSlice(slice* dims)
     def visit_ExtSlice(self, n: ast3.ExtSlice) -> TupleExpr:
-        return TupleExpr(self.translate_expr_list(n.dims))
+        # cast for mypyc's benefit on Python 3.9
+        return TupleExpr(self.translate_expr_list(cast(Any, n.dims)))
 
     # Index(expr value)
     def visit_Index(self, n: Index) -> Node:
-        return self.visit(n.value)
+        # cast for mypyc's benefit on Python 3.9
+        return self.visit(cast(Any, n.value))
 
 
 class TypeConverter: