Fixed queens pep8

- fixed wrong url in constraints url
- enforce use of python2.7 because that was the only supported version
  at that time. Code fails with python3.5

Taken from https://review.opendev.org/#/c/731752

Change-Id: Ieb6a897aa5801ea43bf6a9e2022dadbec3f5164a
Related-Bug: #1881288

tox.ini

@@ -7,7 +7,7 @@ envlist = py27,pep8
 usedevelop = True
 install_command = pip install {opts} {packages}
 deps =
-    -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt?h=stable/queens}
+    -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/queens}
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/test-requirements.txt
 commands=
@@ -20,6 +20,7 @@ commands =
 commands = {posargs}
 
 [testenv:pep8]
+basepython = python2.7
 commands = ./run-flake8
 
 [flake8]