remove python 2.6 trove classifier

OpenStack projects are no longer being tested under Python 2.6, so
remove the trove classifier implying that this project supports 2.6.

Change-Id: I5c3610f0122712f32ac98cc5d859ee9d79e74990

setup.cfg

@@ -16,7 +16,6 @@ classifier =
     Programming Language :: Python
     Programming Language :: Python :: 2
     Programming Language :: Python :: 2.7
-    Programming Language :: Python :: 2.6
 keywords = gerrit console urwid review
 
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