black: upgraded package to upstream

Upgraded dev-python/black to version 21.5_beta1 on amd64

Note: modified PYTHON_COMPAT to reflect that black is compatible with
Python 3.6 (this was not in the upstream package).  Additionally, to
support 3.6, the 3.6-only dependency on the dataclasses backport is
required.

BUG=b:192389533
TEST=emerge and use black to format some code

Change-Id: If6d8016c538c9f6b739e39369d2764f4f3676885
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/portage-stable/+/2998370
Tested-by: Jack Rosenthal <jrosenth@chromium.org>
Reviewed-by: Allen Webb <allenwebb@google.com>
Commit-Queue: Jack Rosenthal <jrosenth@chromium.org>
5 files changed