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Re-implement magic trailing comma handling:

- when a trailing comma is specified in any bracket pair, that signals to Black
  that this bracket pair needs to be always exploded, e.g. presented as "one
  item per line";

- this causes some changes to previously formatted code that erroneously left
  trailing commas embedded into single-line expressions;

- internally, Black needs to be able to identify trailing commas that it put
  itself compared to pre-existing trailing commas. We do this by using/abusing
  lib2to3's `was_checked` attribute.  It's True for internally generated
  trailing commas and False for pre-existing ones (in fact, for all
  pre-existing leaves and nodes).

Fixes #1288

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