Drop Python 2.6 support

Python 2.6 is rather old; this drops support for it entirely.  In
particular, this means we can now drop the dependency on argparse, as
argparse is in the standard library in Python 2.7 and Python 3.3 and
above.

requirements.txt

@@ -1,3 +1,2 @@
-argparse
 setuptools
 six

setup.py

@@ -21,7 +21,7 @@ def readfile(filename):
 
 setup(
     name='cli_tools',
-    version='0.3.0',
+    version='0.4.0',
     author='Kevin L. Mitchell',
     author_email='klmitch@mit.edu',
     url='https://github.com/klmitch/cli_tools',
@@ -34,7 +34,6 @@ def readfile(filename):
         'License :: OSI Approved :: GNU General Public License v3 or '
         'later (GPLv3+)',
         'Operating System :: OS Independent',
-        'Programming Language :: Python :: 2.6',
         'Programming Language :: Python :: 2.7',
         'Programming Language :: Python :: 3.3',
         'Programming Language :: Python :: 3.4',

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py26,py27,py33,py34,pep8
+envlist = py27,py33,py34,pep8
 
 [testenv]
 setenv = LANG=en_US.UTF-8