Use six for python major version detection

setuptools/tests/test_build_ext.py

 import sys
 import distutils.command.build_ext as orig
-
 from distutils.sysconfig import get_config_var
+
+from setuptools.extern import six
+
 from setuptools.command.build_ext import build_ext, get_abi3_suffix
 from setuptools.dist import Distribution
 from setuptools.extension import Extension
@@ -35,7 +37,7 @@ class TestBuildExt:
         assert 'spam.eggs' in cmd.ext_map
         res = cmd.get_ext_filename('spam.eggs')
 
-        if sys.version_info[0] == 2 or not get_abi3_suffix():
+        if six.PY2 or not get_abi3_suffix():
             assert res.endswith(get_config_var('SO'))
         elif sys.platform == 'win32':
             assert res.endswith('eggs.pyd')