Attached is a patch to upgrade pius to a version that works with
gpg-agent 2.1, in my experience.  I have tested this version with
gnupg@2.1.17 as the gpg input and gpg-agent version, and using a new
style keybox (pubring.kbx) file as the keyring.

This also effectively reverts commit 31fbf4b6:

  "gnu: pius: Depend on GnuPG 2.0 rather than 2.1."

Another thread in guix-devel [0] suggests that pius could not deal with
gpg-agent 2.1 which is why it may have been configured to use gnupg-2.0.
The attached patch brings support for 2.1, so I think I am also allowed
to revert 31fbf4b6.

Note, however, the code comment describes a different concern:

> -             ("gpg" ,gnupg-2.0)))          ;2.1 fails to talk to gpg-agent 2.0

I have yet to test gpg/gpgme 2.1 with gpg-agent 2.0, mostly due to the
fact that gnupg 2.0 and 2.1 keep/expect agent sockets in completely
different directories.  I tried a couple of things and lost my patience.
I do not understand how gpg 2.1 can talk to gpg-agent 2.0 without weird
hackery.

This version installs from git because it has been 20 days since my
request to bump the version [1], and I got impatient.

Troy

[0] https://lists.gnu.org/archive/html/guix-devel/2016-08/msg01885.html
[1] https://github.com/jaymzh/pius/issues/46