don't check for failure with missing public keys

GPG actually handles missing public keys in the temporary keyring
correctly: the agent passes those along, so that test was failing in
GPG 2.1.

tests/test_ui.py

@@ -180,9 +180,6 @@ this duplicates tests from the gpg code, but is necessary to test later function
             f.write('default-key 323F39BD')
         self.ui.prepare()
         self.test_copy_secrets()
-        # public key is missing, this should fail
-        with self.assertRaises(GpgRuntimeError):
-            self.ui.sign_key()
         self.ui.keyring.import_data(open(os.path.dirname(__file__) +
                                          '/323F39BD.asc').read())
         self.ui.keyring.import_data(open(os.path.dirname(__file__) +