Remove unnecessary py27 testenv
Remove unnecessary py27 testenv

The pyNN factors automatically set basepython unless explicitly requested otherwise.
Therefore, "testenv:py27" section are unnecessary and should be removed
as the recommendation from Stephen in [1]

[1] https://review.openstack.org/#/c/580250/3/tox.ini@15

Change-Id: I07428041f70c68520c451589fc65cb586741b69b

openstack/stevedore