Incorrect unparsing of extended slices in Python 3.9

[mdanatg]

Repro:

Python 3.9.2 (default, Mar  3 2021, 20:02:32) 
[GCC 7.3.0] :: Anaconda, Inc. on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import astunparse
>>> import ast
>>> n = ast.parse('return foo(x)[:, tf.newaxis] - foo(x)[tf.newaxis, :]')
>>> astunparse.unparse(n)
'\nreturn (foo(x)[(:, tf.newaxis)] - foo(x)[(tf.newaxis, :)])\n'
>>> ast.parse(_)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/mdan/miniconda3/envs/py39test/lib/python3.9/ast.py", line 50, in parse
    return compile(source, filename, mode, flags,
  File "<unknown>", line 2
    return (foo(x)[(:, tf.newaxis)] - foo(x)[(tf.newaxis, :)])
                    ^
SyntaxError: invalid syntax
>>> 

It seems that the extra parens inserted around the slice are at fault. They do not appear in Python 3.7.

[isidentical]

Can verify that this works on upstream ast.unparse

[aiandit]

Hi, I created a pull request that fixes the issue for me with py39

#68