Drop support for EOL Pythons

Support matrix now matches the test matrix from
6233f07bf303d442b39ac677fa6079acc64834a6. By matching these, helps to
ensure all supported Python do indeed work.

Python 2.6 and 3.3 are end of life. They are no longer receiving bug
fixes, including for security issues. Python 2.6 went EOL on 2013-10-29
and on 2017-09-29. For additional details on support Python versions,
see:

Supported: https://devguide.python.org/#status-of-python-branches
EOL: https://devguide.python.org/devcycle/#end-of-life-branches

Removing support for EOL Pythons will reduce testing and maintenance
resources while allowing the library to move towards a modern Python 3
style.

Using pypinfo, we can show the PyPI download statistics, show very low
numbers for EOL Pythons.

| python_version | percent | download_count |
| -------------- | ------: | -------------: |
| 2.7            |  48.94% |         37,036 |
| 3.6            |  26.58% |         20,113 |
| 3.5            |  15.38% |         11,642 |
| 3.4            |   5.93% |          4,488 |
| 3.7            |   3.13% |          2,365 |
| 3.3            |   0.03% |             21 |
| 2.6            |   0.01% |              9 |
| Total          |         |         75,674 |

1 files changed, 2 insertions, 2 deletions

diff --git a/setup.py b/setup.py
index bef9357..963c483 100644
--- a/setup.py
+++ b/setup.py
@@ -5,10 +5,10 @@ setup(
     name='natsort',
     version='5.4.1',
     packages=find_packages(),
-    install_requires=["argparse; python_version < '2.7'"],
     entry_points={'console_scripts': ['natsort = natsort.__main__:main']},
+    python_requires=">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*",
     extras_require={
-        'fast': ["fastnumbers >= 2.0.0; python_version > '2.6'"],
+        'fast': ["fastnumbers >= 2.0.0"],
         'icu': ["PyICU >= 1.0.0"]
     }
 )