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/utils.py b/tests/utils.py
index 226d64f..3e35970 100644
--- a/tests/utils.py
+++ b/tests/utils.py
@@ -15,9 +15,11 @@
 
 import collections
 
+import pytest
+
 import six
 
-import pytest
+import cryptography_vectors
 
 
 HashVector = collections.namedtuple("HashVector", ["message", "digest"])
@@ -66,9 +68,8 @@
 
 
 def load_vectors_from_file(filename, loader):
-    from cryptography import vectors
-    vector_file = vectors.open_vector_file('hazmat', 'primitives', filename)
-    return loader(vector_file)
+    with cryptography_vectors.open_vector_file(filename) as vector_file:
+        return loader(vector_file)
 
 
 def load_nist_vectors(vector_data):