Maybe the problem is not in the code after all

.gitattributes

@@ -0,0 +1 @@
+tests/sample_data/** binary

tests/__init__.py

@@ -28,5 +28,5 @@ tests_datadir = os.path.join(os.path.abspath(os.path.dirname(__file__)),
 def get_testdata(*paths):
     """Return test data"""
     path = os.path.join(tests_datadir, *paths)
-    with open(path, 'rb', newline='') as f:
+    with open(path, 'rb') as f:
         return f.read()