It looks like 0.5.5 (which chromium os currently has) is the latest stable version. 0.6.0 is still testing.

Should we hold off on upgrading this until 0.6.0 is stable, or is there a compelling reason to upgrade this to the unstable version at this time?

Well we can do 0.5.5 for now perhaps

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/overlays/portage-stable/+/73a12495d4e636bc4b68fe0fbfcae3dbbfc00689

commit 73a12495d4e636bc4b68fe0fbfcae3dbbfc00689
Author: Simon Glass <sjg@chromium.org>
Date: Fri Jun 09 16:16:27 2017

pyxattr: upgraded package to upstream

Upgraded dev-python/pyxattr to version 0.5.5 on amd64, arm, x86

BUG= chromium:710119 
TEST=build on reef, daisy, x86-zgb

Change-Id: Ibf21d78967b7dc996aacb46edb9ca9cb7ee7833f
Reviewed-on: https://chromium-review.googlesource.com/527521
Commit-Ready: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jason Clinton <jclinton@chromium.org>

[add] https://crrev.com/73a12495d4e636bc4b68fe0fbfcae3dbbfc00689/metadata/md5-cache/dev-python/pyxattr-0.5.5
[modify] https://crrev.com/73a12495d4e636bc4b68fe0fbfcae3dbbfc00689/dev-python/pyxattr/metadata.xml
[delete] https://crrev.com/b34992707317b1392189916aacde07dc79aa68fe/metadata/md5-cache/dev-python/pyxattr-0.5.3
[modify] https://crrev.com/73a12495d4e636bc4b68fe0fbfcae3dbbfc00689/dev-python/pyxattr/Manifest
[rename] https://crrev.com/73a12495d4e636bc4b68fe0fbfcae3dbbfc00689/dev-python/pyxattr/pyxattr-0.5.5.ebuild