test_ukify: skip test requiring cryptography

I'll add the dep in Fedora spec so it does get tested at least
in some cases.

src/ukify/test/test_ukify.py

@@ -713,6 +713,9 @@ def test_key_cert_generation(tmpdir):
     ])
     assert opts.verb == 'genkey'
     ukify.check_cert_and_keys_nonexistent(opts)
+
+    pytest.importorskip('cryptography')
+
     ukify.generate_keys(opts)
 
     if not shutil.which('openssl'):