dev-python/flatbuffers: Add python-flatbuffers

This library allows us to use flatbuffers in python.

Source:
https://gitweb.gentoo.org/repo/gentoo.git/tree/dev-python/flatbuffers

BUG=b:211717735
TEST=sudo emerge dev-python/flatbuffers

Change-Id: I5ceb094412412ea65e16d047f6e9b37477b15313
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/portage-stable/+/3351693
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Leo Lai <cylai@google.com>
Commit-Queue: Yi Chou <yich@google.com>
Tested-by: Yi Chou <yich@google.com>