[Buildbot] #3488: drop python 2.6 support

[Buildbot trac]

#3488: drop python 2.6 support
------------------------+--------------------
Reporter:  tardyp       |       Owner:
    Type:  enhancement  |      Status:  new
Priority:  major        |   Milestone:  0.9.0
 Version:  master       |  Resolution:
Keywords:               |
------------------------+--------------------

Comment (by rutsky):

 We have at least one active Python 2.6 user @anish:
 https://github.com/buildbot/buildbot/pull/2053#issuecomment-197593687

 > We're running RHEL6 boxes. The build environment on these is being
 recreated using containers. While the base system might move up from
 python2.6, the build environment might take longer to move up. Would be
 nice to have the ability to run against 2.6, even if it is unsupported.

--
Ticket URL: <http://trac.buildbot.net/ticket/3488#comment:3>
Buildbot <http://buildbot.net/>
Buildbot: build/test automation