Force tox to use Py26 for Flake8 env.
Force tox to use Py26 for Flake8 env.

WHen Flake8 is run in Python3, it will fail on "undefined" objects which
are onlu used in if statements that run on Python2. As a workaround,
force tox to use Python2 when running Flake8. This can just be removed
in the future if/when Python2 support is dropped.

pjbull/mkdocs,waylan/mkdocs,lukfor/mkdocs,jimporter/mkdocs,pjbull/mkdocs,williamjmorenor/mkdocs,jimporter/mkdocs,mkdocs/mkdocs,peter1000/mkdocs,mkdocs/mkdocs,dmehra/mkdocs,williamjmorenor/mkdocs,dmehra/mkdocs,waylan/mkdocs,peter1000/mkdocs,lukfor/mkdocs,lukfor/mkdocs,d0ugal/mkdocs,williamjmorenor/mkdocs,jimporter/mkdocs,mkdocs/mkdocs,peter1000/mkdocs,dmehra/mkdocs,d0ugal/mkdocs,d0ugal/mkdocs,pjbull/mkdocs,waylan/mkdocs