Python 2.6: drop support and require Python 2.7 or greater

The main reason for supporting Python 2.6 was the EL6 distros.
Although they are still important, our users are running a combination
of Avocado LTS and Avocado-VT.  That allows us to drop support for
Python 2.6 on master (and further Avocado releases).

Also, with Python 2.6 being dropped as a supported platform, the
version of the stevedore library can be simplified.  Let's reflect
that on both setup.py and requirements*.txt.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

.travis.yml

@@ -2,7 +2,6 @@ language: python
 
 python:
     - "2.7_with_system_site_packages"
-    - "2.6"
 
 branches:
     only:

requirements-travis.txt

@@ -16,8 +16,3 @@ psutil==3.1.1
 # stevedore for loading "new style" plugins
 stevedore==1.8.0
 lxml>=3.4.4
-# All python 2.6 specific requirements (backports)
-argparse==1.3.0; python_version < '2.7'
-logutils==0.3.3; python_version < '2.7'
-importlib==1.0.3; python_version < '2.7'
-unittest2==1.0.0; python_version < '2.7'

requirements.txt

@@ -10,13 +10,6 @@ pyliblzma>=0.5.3
 # REST client (avocado.core.restclient)
 requests>=1.2.3
 # stevedore for loading "new style" plugins
-stevedore>=1.8.0; python_version >= '2.7'
-# stevedore-1.11.0 won't support python2.6 anymore
-stevedore>=1.8.0,<=1.10.0; python_version < '2.7'
-# Additional python 2.6 specific requirements for avocado and unittests (backports)
-argparse>=1.3.0; python_version < '2.7'
-logutils>=0.3.3; python_version < '2.7'
-importlib>=1.0.3; python_version < '2.7'
-unittest2>=1.0.0; python_version < '2.7'
+stevedore>=1.8.0
 # Aexpect is now used on Docker plugin
 aexpect>=1.2.0

setup.py

@@ -24,11 +24,6 @@ from setuptools import setup, find_packages
 VERSION = open('VERSION', 'r').read().strip()
 VIRTUAL_ENV = hasattr(sys, 'real_prefix')
 
-if sys.version_info[:2] >= (2, 7):
-    stevedore_version = 'stevedore>=1.8.0'
-else:
-    stevedore_version = 'stevedore>=1.8.0,<=1.10.0'
-
 
 def get_dir(system_path=None, virtual_path=None):
     """
@@ -175,5 +170,5 @@ if __name__ == '__main__':
               },
           zip_safe=False,
           test_suite='selftests',
-          python_requires='>=2.6',
-          install_requires=[stevedore_version])
+          python_requires='>=2.7',
+          install_requires=['stevedore>=1.8.0'])