Fix parser when using Python 3.9

The structure of the AST for subscripts was changed in
python/cpython#9605

Fixes #8627

mypy/fastparse.py

@@ -49,6 +49,8 @@
         import ast as ast3
         assert 'kind' in ast3.Constant._fields, \
                "This 3.8.0 alpha (%s) is too old; 3.8.0a3 required" % sys.version.split()[0]
+        # TODO: Num, Str, Bytes, NameConstant, Ellipsis are deprecated in 3.8.
+        # TODO: Index, ExtSlice are deprecated in 3.9.
         from ast import (
             AST,
             Call,
@@ -1505,17 +1507,22 @@ def visit_Bytes(self, n: Bytes) -> Type:
 
     # Subscript(expr value, slice slice, expr_context ctx)
     def visit_Subscript(self, n: ast3.Subscript) -> Type:
-        if not isinstance(n.slice, Index):
-            self.fail(TYPE_COMMENT_SYNTAX_ERROR, self.line, getattr(n, 'col_offset', -1))
-            return AnyType(TypeOfAny.from_error)
+        if sys.version_info >= (3, 9):  # Really 3.9a5 or later
+            sliceval = n.slice  # type: Any
+        else:
+            # Python 3.8 or earlier use a different AST structure for subscripts
+            if not isinstance(n.slice, Index):
+                self.fail(TYPE_COMMENT_SYNTAX_ERROR, self.line, getattr(n, 'col_offset', -1))
+                return AnyType(TypeOfAny.from_error)
+            sliceval = n.slice.value
 
         empty_tuple_index = False
-        if isinstance(n.slice.value, ast3.Tuple):
-            params = self.translate_expr_list(n.slice.value.elts)
-            if len(n.slice.value.elts) == 0:
+        if isinstance(sliceval, ast3.Tuple):
+            params = self.translate_expr_list(sliceval.elts)
+            if len(sliceval.elts) == 0:
                 empty_tuple_index = True
         else:
-            params = [self.visit(n.slice.value)]
+            params = [self.visit(sliceval)]
 
         value = self.visit(n.value)
         if isinstance(value, UnboundType) and not value.args: