gnu: python-gpg: Use explicit version 1.8.0 instead of GPGME's.

GPGME defines version 1.9.0, which isn't yet available for python-gnupg, whose
latest version is 1.8.0, so we use that explicitly instead.

Fixes <https://bugs.gnu.org/26610

>.

* gnu/packages/gnupg.scm (python-gpg, python2-gpg): Use explicit version 1.8.0
instead of GPGME's version.

Signed-off-by: Leo Famulari <leo@famulari.name>