Describe the bug
When running black with -t py35 code that raises syntax errors is left unchanged.

To Reproduce:

example.py

def func(
    a_long_variable_name: int = 1000000,
    another_long_variable_name: int = 1000000,
    **kwargs,
):
    pass

This example is valid in python 3.7 but not in python 3.5.

1. Run Black on it like so:

   black --target-version=py35 example.py

2. File remains unchanged

Expected behavior
I would expect black to tell me that the trailing comma is a syntax error in the target python version, or remove the trailing comma.

Environment:

• black: 19.10b0
• OS: Docker using python:3.7-buster (Python 3.7.7 (default, May 16 2020, 07:16:36))

Does this bug also happen on master?
Yes

Additional context
Maybe related #419 ?