Drop support for EOL Pythons (#131)

Python 2.4, 2.5, 2.6, 3.2 and 3.3 are end of life. They are no longer
receiving bug fixes, including for security issues. These Pythons went
EOL on:

Python 2.6: 2013-10-29
Python 3.2: 2016-02-20
Python 3.3: 2017-09-29

For additional details on supported Python versions, see:

https://devguide.python.org/#status-of-python-branches

Removing support for EOL Pythons will reduce testing and maintenance
resources.

Using pypinfo, here are the download statistics of the project for the
last 30 days, showing limited downloads from EOL Pythons:

| python_version | percent | download_count |
| -------------- | ------: | -------------: |
| 2.7            |  90.33% |      7,975,885 |
| 3.6            |   4.95% |        437,261 |
| 3.5            |   2.99% |        263,637 |
| 3.4            |   1.02% |         89,723 |
| 2.6            |   0.69% |         60,511 |
| 3.7            |   0.02% |          2,024 |
| 3.3            |   0.00% |            441 |
| 3.2            |   0.00% |             38 |
| 3.8            |   0.00% |              9 |
| None           |   0.00% |              2 |

By removing support for older Pythons, can remove or update the
following workarounds and code:

* Built-in function bin() is always available.
* Built-in function callable() is always available.
* Built-in function all() is always available.
* The collections.OrderedDict class is always available.
* The datetime.strptime() method is always available.
* The platform module is always available.
* The string.partition() method is always available.
* The NullHandler class is always available.
* The method .__trunc__() is always available.
* Can use set literals.
* Updated unittest is always available. The unittest2 module is
  unnecessary.

Users on older versions of Python can continue to download, install, and
use older versions of the library that continue to have support for
older Pythons.

1 files changed, 5 insertions, 14 deletions

diff --git a/setup.py b/setup.py
index fa0a876..1992835 100644
--- a/setup.py
+++ b/setup.py
@@ -7,6 +7,7 @@
 #
 import os
 import sys
+import unittest
 
 classifiers = """\
 Development Status :: 5 - Production/Stable
@@ -20,13 +21,8 @@ License :: OSI Approved :: BSD License
 Natural Language :: English
 Operating System :: OS Independent
 Programming Language :: Python :: 2
-Programming Language :: Python :: 2.4
-Programming Language :: Python :: 2.5
-Programming Language :: Python :: 2.6
 Programming Language :: Python :: 2.7
 Programming Language :: Python :: 3
-Programming Language :: Python :: 3.2
-Programming Language :: Python :: 3.3
 Programming Language :: Python :: 3.4
 Programming Language :: Python :: 3.5
 Programming Language :: Python :: 3.6
@@ -49,8 +45,8 @@ def howto_install_setuptools():
 """)
 
 
-if sys.version_info[:2] < (2, 4):
-    print("ERROR: this package requires Python 2.4 or later!")
+if sys.version_info[:2] < (2, 7):
+    print("ERROR: this package requires Python 2.7 or later!")
     sys.exit(1)
 
 try:
@@ -88,13 +84,8 @@ params.update({
                  'pyasn1.codec.ber',
                  'pyasn1.codec.cer',
                  'pyasn1.codec.der',
-                 'pyasn1.codec.native']})
-
-# handle unittest discovery feature
-try:
-    import unittest2 as unittest
-except ImportError:
-    import unittest
+                 'pyasn1.codec.native'],
+    'python_requires': '>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*'})
 
 
 class PyTest(Command):