travis-ci: revert PYTHONPATH settings
lint: fix py3 compat after updated with 2to3

This should fix pylint runs under py3 on travis-ci.
Merge branch 'master' of git://github.com/vapier/snakeoil

More pylint fixes (closes #3)
setup: drop py2.6 support
lint: remove unnecessary ignore shadowing for 'next'

This was previously dropped from compat support when pre-py2.6 support
was dropped.
lint: drop deprecated include-ids option

This is ignored by newer pylint versions.
lint: add spaces for multiline lint messages
pylint: skip duplicate code checks
pylint: update const node checking

Newer pylint marks nodes constant with the class astroid.node_classes.Const
while older versions use logilab.astng.node_classes.Const.  Check both now.
lint: fix spacing around operators