Move cryptography.vectors to cryptography_vectors

All vectors are now stored in the subpackage in the vectors/ folder.

This package is automatically installed by setup.py test and will also
be uploaded with a matching version number by the PyPI upload task.
diff --git a/tests/test_fernet.py b/tests/test_fernet.py
index 438dc09..34760e3 100644
--- a/tests/test_fernet.py
+++ b/tests/test_fernet.py
@@ -28,15 +28,17 @@
 from cryptography.hazmat.backends import default_backend
 from cryptography.hazmat.primitives.ciphers import algorithms, modes
 
+import cryptography_vectors
+
 
 def json_parametrize(keys, filename):
-    from cryptography import vectors
-    vector_file = vectors.open_vector_file('fernet', filename)
-    data = json.load(vector_file)
-    return pytest.mark.parametrize(keys, [
-        tuple([entry[k] for k in keys])
-        for entry in data
-    ])
+    vector_file = cryptography_vectors.open_vector_file('fernet', filename)
+    with vector_file:
+        data = json.load(vector_file)
+        return pytest.mark.parametrize(keys, [
+            tuple([entry[k] for k in keys])
+            for entry in data
+        ])
 
 
 @pytest.mark.cipher