alter vector file loader to support passing read mode
diff --git a/tests/test_fernet.py b/tests/test_fernet.py
index 455d985..5b7a9f9 100644
--- a/tests/test_fernet.py
+++ b/tests/test_fernet.py
@@ -7,6 +7,7 @@
 import base64
 import calendar
 import json
+import os
 import time
 
 import iso8601
@@ -24,7 +25,9 @@
 
 
 def json_parametrize(keys, filename):
-    vector_file = cryptography_vectors.open_vector_file('fernet', filename)
+    vector_file = cryptography_vectors.open_vector_file(
+        os.path.join('fernet', filename), "r"
+    )
     with vector_file:
         data = json.load(vector_file)
         return pytest.mark.parametrize(keys, [