Remove py27 from tox env list

cyborg job already been migrated to python3
in I79154110bb13ffa173b544e88bf87fc9de79979f

This commit remove py27 from tox env list too.

Change-Id: Ib4611280bc0fa3139e6ebf7b0d467ad11ff75f80

tox.ini

@@ -1,9 +1,11 @@
 [tox]
-minversion = 2.0
-envlist = py35,py27,pep8
+minversion = 3.1.1
+envlist = py37,pep8
 skipsdist = True
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 install_command = pip install {opts} {packages}
 setenv =
@@ -17,15 +19,12 @@ deps = -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/
 commands = stestr run {posargs}
 
 [testenv:pep8]
-basepython = python3
 commands = flake8 {posargs}
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:cover]
-basepython = python3
 setenv =
     VIRTUAL_ENV={envdir}
     PYTHON=coverage run --source cyborg-tempest-plugin --parallel-mode
@@ -36,18 +35,15 @@ commands =
     coverage xml -o cover/coverage.xml
 
 [testenv:docs]
-basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:releasenotes]
-basepython = python3
 deps = {[testenv:docs]deps}
 commands =
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:debug]
-basepython = python3
 commands = oslo_debug_helper {posargs}
 
 [flake8]