Add system's requirements.txt, enforce flake8 linter

Fix style issues in functional tests.

system/t07_serve/__init__.py

@@ -8,10 +8,9 @@ import signal
 import subprocess
 import shlex
 import time
-import errno
 
 from lib import BaseTest
-from socket import error as socket_error
+
 
 class RootDirInaccessible(BaseTest):
     """
@@ -21,12 +20,13 @@ class RootDirInaccessible(BaseTest):
     fixturePool = False
 
     configOverride = {
-        "rootDir": "/root" # any directory that exists but is not writable
+        "rootDir": "/root"  # any directory that exists but is not writable
     }
 
     runCmd = "aptly serve -listen=127.0.0.1:8765"
     expectedCode = 1
 
+
 class Serve1Test(BaseTest):
     """
     serve public: two publishes, verify HTTP