Replace use of deprecated cert.not_valid_after

... attribute. See [1]

[1]: https://cryptography.io/en/latest/x509/reference/#cryptography.x509.Certificate.not_valid_after

Change-Id: I3dd573300a8dfbe68eaeb736b47f6abb61b5e85b

Author: weinimo

octavia/common/tls_utils/cert_parser.py

@@ -300,7 +300,7 @@ def get_cert_expiration(certificate_pem):
     try:
         cert = x509.load_pem_x509_certificate(certificate_pem,
                                               backends.default_backend())
-        return cert.not_valid_after
+        return cert.not_valid_after_utc
     except Exception as e:
         LOG.exception('Unreadable Certificate.')
         raise exceptions.UnreadableCert from e

octavia/tests/unit/common/tls_utils/test_cert_parser.py

@@ -246,7 +246,10 @@ def test_get_primary_cn(self):
 
     def test_get_cert_expiration(self):
         exp_date = cert_parser.get_cert_expiration(sample_certs.X509_EXPIRED)
-        self.assertEqual(datetime.datetime(2016, 9, 25, 18, 1, 54), exp_date)
+        self.assertEqual(
+            datetime.datetime(2016, 9, 25, 18, 1, 54,
+                              tzinfo=datetime.timezone.utc),
+            exp_date)
 
         # test the exception
         self.assertRaises(exceptions.UnreadableCert,