Add system's requirements.txt, enforce flake8 linter

Fix style issues in functional tests.

system/t05_snapshot/diff.py

@@ -2,6 +2,10 @@ import re
 from lib import BaseTest
 
 
+def trimTrailingWhitespace(_, s):
+    return re.sub(r'\s*$', '', s, flags=re.MULTILINE)
+
+
 class DiffSnapshot1Test(BaseTest):
     """
     diff two snapshots: normal diff
@@ -13,8 +17,7 @@ class DiffSnapshot1Test(BaseTest):
         "aptly snapshot pull snap1 snap2 snap3 'rsyslog (>= 7.4.4)'"
     ]
     runCmd = "aptly snapshot diff snap1 snap3"
-    # trim trailing whitespace
-    outputMatchPrepare = lambda _, s: re.sub(r'\s*$', '', s, flags=re.MULTILINE)
+    outputMatchPrepare = trimTrailingWhitespace
 
 
 class DiffSnapshot2Test(BaseTest):
@@ -27,8 +30,7 @@ class DiffSnapshot2Test(BaseTest):
         "aptly snapshot create snap2 from mirror wheezy-backports",
     ]
     runCmd = "aptly snapshot diff snap1 snap2"
-    # trim trailing whitespace
-    outputMatchPrepare = lambda _, s: re.sub(r'\s*$', '', s, flags=re.MULTILINE)
+    outputMatchPrepare = trimTrailingWhitespace
 
 
 class DiffSnapshot3Test(BaseTest):
@@ -41,8 +43,7 @@ class DiffSnapshot3Test(BaseTest):
         "aptly snapshot create snap2 from mirror wheezy-backports",
     ]
     runCmd = "aptly snapshot diff -only-matching snap1 snap2"
-    # trim trailing whitespace
-    outputMatchPrepare = lambda _, s: re.sub(r'\s*$', '', s, flags=re.MULTILINE)
+    outputMatchPrepare = trimTrailingWhitespace
 
 
 class DiffSnapshot4Test(BaseTest):