Use flake8 to find Python syntax errors or undefined names

.travis.yml

@@ -7,7 +7,13 @@ python:
 - pypy
 
 install:
-    - travis_retry pip install -q pytest mock
+    - travis_retry pip install -q flake8 pytest mock
+
+before_script:
+    # stop the build if there are Python syntax errors or undefined names.
+    - if [[ $TRAVIS_PYTHON_VERSION != '2.6' ]]; then flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics; fi
+    # exit-zero treats all errors as warnings.  The GitHub editor is 127 chars wide.
+    - if [[ $TRAVIS_PYTHON_VERSION != '2.6' ]]; then flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics; fi
 
 script:
   - PYTHONPATH=. py.test

sshuttle/assembler.py

@@ -2,9 +2,9 @@ import sys
 import zlib
 import imp
 
+verbosity = verbosity  # noqa: F821 must be a previously defined global
 z = zlib.decompressobj()
 while 1:
-    global verbosity
     name = sys.stdin.readline().strip()
     if name:
         name = name.decode("ASCII")