Backport latest black changes (Python 3.9)

[MatthewScholefield]

I noticed blue is failing for a specific Python 3.9 syntax:

def get_dec_dec():
  def get_dec():
    def dec(func):
      print(func)
    return dec
  return get_dec

@get_dec_dec()()
def foo():
  ...

error: cannot format example.py: Cannot parse: 8:14: @get_dec_dec()()
Oh no! 💥 💔 💥
1 file failed to reformat

It would be nice to support this so I was wondering if it would be possible to port changes from the latest black to blue.

[grantjenks]

Fixed in #54