Remove Tox from test_requires

This commit removes tox from the `test_requires` section in 'setup.py',
since it isn't actually necessary to run the tests.

Author: DirectXMan12

setup.py

@@ -261,8 +261,5 @@ def gssapi_modules(lst):
         'enum34',
         'decorator',
         'six'
-    ],
-    tests_require=[
-        'tox'
     ]
 )