More CLI tests & clearer bytes stuff

Ensuring that bytes are written correctly on all supported Python
versions, including when writing to stdout.
diff --git a/tests/test_load_save_keys.py b/tests/test_load_save_keys.py
index 6f374cf..0caa067 100644
--- a/tests/test_load_save_keys.py
+++ b/tests/test_load_save_keys.py
@@ -14,7 +14,7 @@
 #  See the License for the specific language governing permissions and
 #  limitations under the License.
 
-'''Unittest for saving and loading keys.'''
+"""Unittest for saving and loading keys."""
 
 import base64
 import unittest
@@ -89,6 +89,7 @@
         key = rsa.key.PrivateKey(3727264081, 65537, 3349121513, 65063, 57287)
         der = key.save_pkcs1('DER')
 
+        self.assertIsInstance(der, bytes)
         self.assertEqual(PRIVATE_DER, der)
 
     def test_load_public_key(self):
@@ -105,6 +106,7 @@
         key = rsa.key.PublicKey(3727264081, 65537)
         der = key.save_pkcs1('DER')
 
+        self.assertIsInstance(der, bytes)
         self.assertEqual(PUBLIC_DER, der)
 
 
@@ -125,6 +127,7 @@
         key = rsa.key.PrivateKey(3727264081, 65537, 3349121513, 65063, 57287)
         pem = key.save_pkcs1('PEM')
 
+        self.assertIsInstance(pem, bytes)
         self.assertEqual(CLEAN_PRIVATE_PEM, pem)
 
     def test_load_public_key(self):
@@ -141,6 +144,7 @@
         key = rsa.key.PublicKey(3727264081, 65537)
         pem = key.save_pkcs1('PEM')
 
+        self.assertIsInstance(pem, bytes)
         self.assertEqual(CLEAN_PUBLIC_PEM, pem)
 
     def test_load_from_disk(self):