Add system's requirements.txt, enforce flake8 linter

Fix style issues in functional tests.

system/t05_snapshot/__init__.py

@@ -1,15 +1,3 @@
 """
 Testing snapshot management
 """
-
-from .create import *
-from .show import *
-from .list import *
-from .verify import *
-from .pull import *
-from .diff import *
-from .merge import *
-from .drop import *
-from .rename import *
-from .search import *
-from .filter import *