Drop unsupported Python 3.3

See: https://en.wikipedia.org/wiki/CPython#Version_history
diff --git a/.travis.yml b/.travis.yml
index d91198c..fcb8f8e 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -10,8 +10,6 @@
       env: TOXENV=py26
     - python: 2.7
       env: TOXENV=py27
-    - python: 3.3
-      env: TOXENV=py33
     - python: 3.4
       env: TOXENV=py34
     - python: 3.5
diff --git a/tox.ini b/tox.ini
index 9d21df6..d673dfb 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,5 +1,5 @@
 [tox]
-envlist = py26,py27,pypy,py33,py34,py35,py36,py37
+envlist = py26,py27,pypy,py34,py35,py36,py37
 
 [testenv]
 deps =