Fix lint in unit tests re: py3-first and py2 compat

Change-Id: I22d359df04f613c9a3306b8eae5ab9298acc9120

.stestr.conf

@@ -0,0 +1,3 @@
+[DEFAULT]
+test_path=./unit_tests
+top_dir=./

unit_tests/__init__.py

@@ -24,5 +24,6 @@ def _add_path(path):
     if path not in sys.path:
         sys.path.insert(1, path)
 
+
 _add_path(_actions)
 _add_path(_hooks)

unit_tests/test_utils.py

@@ -150,7 +150,7 @@ def patch_open():
 
     Yields the mock for "open" and "file", respectively.'''
     mock_open = MagicMock(spec=open)
-    mock_file = MagicMock(spec=file)
+    mock_file = MagicMock(spec=__file__)
 
     @contextmanager
     def stub_open(*args, **kwargs):