drop python2.6 support

The Python core team[*1] stopped supporting python 2.6. Some Python
libraries started following the same path and trying to support 2.6
becomes painful...

For example, most of you already noticed, oslo.config dropped 2.6
support. I tried to work around it with the following commit:

2250171

But this forces users of decent distributions (e.g Ubuntu 14.04) to
upgrade pip (old pip versions can't understand version conditions).

[*1]
http://www.curiousefficiency.org/posts/2015/04/stop-supporting-python26.html

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>

.travis.yml

@@ -3,7 +3,6 @@ python:
   - "2.7"
 
 env:
-  - TOX_ENV=py26
   - TOX_ENV=py27
   - TOX_ENV=py34
   - TOX_ENV=pypy

setup.cfg

@@ -13,7 +13,6 @@ classifier =
     Topic :: System :: Networking
     Natural Language :: English
     Programming Language :: Python
-    Programming Language :: Python :: 2.6
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.4

tools/pip-requires

@@ -1,8 +1,7 @@
 eventlet>=0.15
 msgpack-python>=0.3.0  # RPC library, BGP speaker(net_cntl)
 netaddr
-oslo.config>=1.6.0, <=3.0.0 ; python_version < '2.7'
-oslo.config>=1.6.0 ; python_version >= '2.7'
+oslo.config>=1.6.0
 routes  # wsgi
 six>=1.4.0
 webob>=1.2  # wsgi

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py26,py27,py34,pypy,pep8
+envlist = py27,py34,pypy,pep8
 
 [testenv]
 deps = -U