Add system's requirements.txt, enforce flake8 linter

Fix style issues in functional tests.

system/t12_api/__init__.py

@@ -1,13 +1,3 @@
 """
 Testing aptly REST API
 """
-
-from .repos import *
-from .files import *
-from .publish import *
-from .version import *
-from .graph import *
-from .snapshots import *
-from .packages import *
-from .unix_socket import *
-from .systemd_handover import *