Drop py33 support

"Python 3.3 support is being dropped since OpenStack Liberty."
written in following URL.
https://wiki.openstack.org/wiki/Python3

And already the infra team and the oslo team are dropping py33
support from their projects.

Since we rely on oslo for a lot of our work, and depend on infra
for our CI, we should drop py33 support too.

Change-Id: I73a53c2c16b3bc8d7cf2c2e491e3be4c4d007828
Closes-Bug: #1526170

setup.cfg

@@ -15,7 +15,7 @@ classifier =
     Programming Language :: Python :: 2
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.3
+    Programming Language :: Python :: 3.4
 
 [files]
 packages = ironicclient

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 1.6
-envlist = py27,py33,py34,pep8,pypy
+envlist = py27,py34,pep8,pypy
 skipsdist = True
 
 [testenv]