licenses: Add Zope Public License.

* guix/licenses.scm (zpl2.1): New variable.

guix/licenses.scm

@@ -61,6 +61,7 @@
             sleepycat
             vim
             x11 x11-style
+            zpl2.1
             zlib
             fsf-free))
 
@@ -382,6 +383,11 @@ which may be a file:// URI pointing the package's tree."
             "Check the URI for details.  "
             comment)))
 
+(define zpl2.1
+  (license "Zope Public License 2.1"
+           "http://directory.fsf.org/wiki?title=License:ZopePLv2.1"
+           "https://www.gnu.org/licenses/license-list.html#Zope2.0"))
+
 (define zlib
   (license "Zlib"
            "http://www.gzip.org/zlib/zlib_license.html"