Enable checking for pep8 confirmance in tox

Also make sure to ignore E741 naming a variable as l will raise this error and
reason is l is similar to 1 and people might get confused. For me it doesn't
look like an error hence ignored in tox.ini

tox.ini

@@ -4,7 +4,7 @@
 # and then run "tox" from this directory.
 
 [tox]
-envlist = {py27,py34,py35,py36,pypy}
+envlist = {py27,py34,py35,py36,pypy,flake8}
 skip_missing_interpreters = True
 minversion = 2.4.0
 
@@ -36,3 +36,12 @@ commands =
     wormhole --version
     coverage run --branch -m wormhole.test.run_trial {posargs:wormhole}
     coverage xml
+
+[testenv:flake8]
+deps = flake8
+commands = flake8 src/wormhole
+
+[flake8]
+ignore = E741
+exclude = .git,__pycache__,docs/source/conf.py,old,build,dist
+max-complexity = 10